Capitals and accent marks indicate the octaves.  The first letter of a tuning name is usually the bass string, then the Middle Drone, lastly the Melody string(s).  CGG/CGc and DAA/DAd are the most common tunings.

C D E F G A B c d e f g a b

The Bass string is usually C or D.  The Middle Drone is then G or A.  If you are tuning to CGG or DAA, the melody string(s) ae the same as the Middle Drone.  If you are tuning CGc or DAd, the Melody string(s) are an octave higher in pitch -- as indicated by the lower case letters c or d.  If you refer to the line of letters above, and count 8 letters to the right of C or D, you will find c or d.   

The system goes both higher and lower than shown here, with accent marks indicating other octaves.

Here Traildad: http://www.strothers.com/string_choice.html

(Make sure you are calculating in the right octave.)

Ken's point is  I think: "one size down.. from what?"- because there are all kinds of different gauge sets for dulcimers that people use, and it also depends on the tunings you want to use.

All that said, a .022 for the wound bass and a .014 for the middle string would likely work fine too. But I'd hesitate to put .012 on the melody string and tune it up to high D on a 29" VSL... it would probably be ok, but it'd be pretty tight and therefore not as comfy to play.  .010s would be perfect for melody strings on a 29"er.  :)

Not necessarily "one size down".  The Strothers string gauge calculator shows that for a 29" VSL tuned DAA/DAd or CGG/CGc, you want .010 for the two Melody strings, a .012 for the Middle Drone, and a .020 wound string.
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It looks totally great!  It has the 6.5 extra fret, btw. I think you'll find that comes in handy.  Don't put too heavy a set of strings on it, considering it's a 29" VSL (vibrating string length).  Maybe .010 for the melody course.

Looks like a good place for you to start!

I borrowed a dulcimer from my sister. I think it’s a kit dulcimer with a 29” VSL. It’s missing strings so I don’t know how it plays yet. It will give me something to work on my music theory with until I buy a dulcimer. .

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