@Dusty, there are 3 octaves across all three strings, 0-6 on the bass then 0-14 [DAd] in the melody [and various combos using the middle string]. Three on one string is something else, probably huge to be usable.

@Skip, you are correct that the way most luthiers make their dulcimers, a longer VSL just means larger spaces in between fret wire, for the fretboard is usually fixed at a little more than two octaves (most have 17, 18, or 19 frets).  However, there is no reason that has to be the case.  A dulcimer with a long enough VSL could be configured for 3 full, usable octaves. 

I personally would have no use for such a dulcimer, but it is theoretically possible to build one.

This is an example of how different our approaches to playing and therefore our design preferences can be.  One reason I love my McCafferty dulcimer is that it has only 14 frets but an extended strum hollow, allowing the right hand a more comfortable angle and more room to play.  I also had the luthier bring the strings closer together for me, for I find it easier to play fast when the strings are closer together.  And the 25" scale length means I can fret chords with ease.

@Barnjam clearly has different needs than I.  He is seeking a dulcimer with a longer VSL to accommodate an extra octave and greater space between the strings to accommodate his large hands.

I forgot to say you would need 17 frets, not counting 1/2 frets to get 2 octaves A [low] to D [high] on one string.

I've found the experts! I'm impressed with all the good ideas and knowledge amongst you. I find this musical journey to be quite fascinating. There are so many factors that can affect the music we play.

I'm a bit confused by all of this. Fourteen frets is two diatonic octaves, D to D or A to A or whatever, regardless of the VSL. A longer VSL [fretboard] just increases the distance between frets. It doesn't 'add' any any notes. A wider fretboard increases space between strings, but so can adjusting the middle string closer to the bass string. My recommendation would be to get something with a long VSL and adjust the string spacing between the middle and bass string. A nut set up for single or double melody strings has a wider space between these two strings built in [usually about 1/8"] by removing the inner melody string.

@barnjam ...remember you don't 'have to' play in the key of D, either. And having a chromatic means you don't have to worry about the usual diatonic 'gaps' in the fretboard when choosing your tunings and keys. 

Yikes.  I once started using .009 strings to tune to dulcimer high D (D4) on a 28" scale. The strings tuned to the note ok, but they were so thin that they broke all too often often while playing. So I went back to using .010 for my melody strings to tune to high D.  But i can't even imagine doing that with a 35" scale length (!) and a .007 string. 
Does that "Stringjoy" Tension Calculator tell you when one of its suggestions would likely result in a string that breaks, or a string that is too slack?  I doubt it includes such useful info. I suspect that like any non-human, all it will inform you of is what numbers are correct for the calculation.  BUT... if anyone could actually do that experiment Nate, it would be YOU!    

More good information to consider. I think some combination of DAA tuning with a wider fretboard would be the best solution, without getting into expensive custom builds. I appreciate your time and knowledge on this.

Strumelia could you please elaborate on that?

The "stringjoy string tension calculator" says that a 35" VSL tuned to D4 with a 0.007 gauge string (lowest Ive seen in an ernie ball pack) will have ~13 lb of tension. 

Another factor when making the fretboard longer and longer is that, depending on what notes you want to tune the strings to, you will have to choose string gauges that will enable you to tune to those notes on that length of fretboard. The trick is that although you'll have a range of choices by choosing various gauges, the choices are not unlimited as to how heavy(low) and how thin(high) you can go for a particular length of fretboard and choice of open string note. You can't for example tune to a dulcimer high D on a fretboard that is 35" long... the required string would be so thin it would snap under tension long before being tuned to high d. The opposite is true as well (trying to tune to very low notes on a very short fretboard... the required string would be impossibly fat and slack).

Yes, I do prefer to only play on the melody string although I have fun playing melody on the bass string sometimes. I have a chromatic Weissenborn, and I enjoy it, but it forces me to think more. My diatonic dulcimers are fun to play because my precision can be lower, with less frets. Thanks Nate. All great ideas.

I think an important question is: do you prefer to only play on the melody string, or would you prefer to be able to fret all three strings?
If you're only fretting the melody string out of compromise, then a wider fretboard could solve your problem.

A dulcimer does not have all the frets of the chromatic scale, which is why the major scale must start on either the open string for DAd (1-5-8) or the 3rd fret for DAA (1-5-5) if you want to have all the notes in the major scale. For this reason, if you wanted to tune the melody string lower than A and still get all the notes of the D major scale, you would have to add extra frets. For example, if you tuned the instrument DAG, there would be no fret for C#, unless you add a 3.5 fret. You could potentially set the fret layout to allow your melody string to be tuned to any note, though it would look strange for some. Or with a chromatic instrument, you could just tune it any old way and always have all the needed frets for your D major scale.

It's possible to tune the melody string lower and add extra frets, but could lead to it's own problems, like if you wanted to play melody on the bass string, your melody string would no longer be a harmonious drone to accompany it.

I see that the dulcimer you showed a photo of has two full octaves of frets plus 3 frets of the third octave. If you have large fingers, it will definitely be difficult to try to fret in those teeny tiny spaces, and if you add more frets to the high end, the gaps between them only get smaller.

The longer the VSL, the more space you'll have between frets in that third octave, but that can only help so much.

That makes perfect sense. The open string note is the lowest a string can be. I'm overthinking it for sure. The solution then is to tune in DAA and add more frets to the upper neck. A larger dulcimer body with a longer neck and wider fretboard would help make the spaces. THANK YOU!

I wrote a blog post on the angling of our fretting fingers that elaborates on that subject a little more- you might find something helpful in it:
https://dulcimer-noter-drone.blogspot.com/2012/07/lazy-fingers.html

Truthfully, I might be misunderstanding your request or your concept here. The bottom line though is that on any string, you cannot play notes lower than that particular string is tuned to... no matter how long the neck is or how many additional frets you put on.

Sorry....couldn't help myself....

Thanks for the responses, all. John, that "Uncle Eddie" is insanely cool. That's a great suggestion on the long neck banjo. I've reached out to John Knopf with these questions. Dusty, agreed and that is exactly what my Milford Blevins model has (in order to accommodate 8 strings). Strumelia, your suggestion of arching the fingers is excellent, and I will try to improve on that. Regarding your questions: 1) Yes, on both a longer and wider fretboard for finger space and tonal range 2) Yes, I'd like to play notes that dip below the "home" note of D on the melody string. I did try DAA tuning and I enjoyed the ability to hit those lower notes, but I was then limited on the higher octave. I'm looking for both sides of the spectrum.

My primary concert dulcimers are modified to be loud. Both my converted guitar/dulcimer, Milford B. model, and J. McAnulty Weissenborn are very resonant even without an amp. I've added drop A & D strings for deep bass and extra resonance. I use Fender heavy triangle picks and higher gauge strings as I tend to strum and pick aggressively. I've attached a few pics to help illustrate the set-ups.

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One added thing I'd like to just check on-
you say you have trouble with bumping into other strings when you fret and slide with your "large hands", yet your dulcimer has a larger than usual fretboard already.
Are you fretting with the bony TIPS of your fingers, with your hand arched up over the strings, or are you fretting with the fatty PADS of your fingers, with your hand held more horizontally as though you are typing on a laptop?  Fretting with te very tips of the fingers can make all the difference in the world when it comes to getting clean sound and not bumping into other strings or muffling your own notes while cutting off resonating notes.

I'm a little confused here and i have questions. You say you like to play the melody mostly all on your melody string while occasionally plucking the middle string, yet you also say you like to play melody on the lower string. 
Are you simply wanting a longer and wider fretboard, and will continue to tune to DAd and play in the key of D?...are you just seeking extra space for your large fingers?
OR... are you wanting to play notes that dip below the 'home' note of D on your melody string, as in tuning to DAA for example?
From your question, I'm not really sure of your goal.

Another possibility would be a wider fretboard with more space in between strings.  Just a thought.

Sending a "long vsl" chart borrowed from Folkcraft...31 3/8 VSL. Is this what you envision? It would take some time to do the math on string gauge etc....To think of what that length looks like, picture a Pete Seger long neck banjo. All you have to do is remove the resonator and slap the remains on an hourglass body....voila!

This other fella on FOTMD may be able to help....looks like he builds some big honkin' dulcimores....

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This is a question regarding dulcimer build limitations. Is it possible to make an extended range dulcimer? I am envisioning a longer fretboard that would allow for several more frets on the lower end. This would allow the player to stay on the melody string for the low and high octave, without crossing over to the middle string, in DAD tuning.

I am a player with large hands, which makes it difficult to play the middle string without hitting other strings. So far, the Milford Blevins Dulcimer has the largest fretboard I’ve used. It is both wider and longer than a standard dulcimer. For me, that means my fat fingers have plenty of space to slide. I wish it were a tad longer so that I could get the best of DAA and DAD tuning, allowing me to walk down the fretboard on the melody string alone. I play melody exclusively on the first D string, with an occasional pluck of the open A middle string. The middle and lower strings I use for droning rhythms, or to play bass melody.

Thank you for any responses.

Mike