I have a different perspective.  Going back to my old career as an engineer and quality control manager, we used a term called "stacked tolerances".  This means that if you have for instance three parts and each is near the outside acceptance criteria of their measurement (although still within the acceptable specified tolerance), the combined effect when the parts are assembled together is that all the "tolerances" are added together to produce a less than optimum part.

So while as Ken states the maximum distance between frets on a 28" and 25" scale may only be 0.2", when you play a chord you have to add ALL those "tolerances" together.  So if you play a Bm chord on the 5th, 3rd and 2nd fret in DAD tuning, the difference in distance is 1/2".  That is a pretty big difference in stretch for someone with small hands.

If I play a 28" dulcimer for ten minutes or so, then switch to a say 25" or 26" I notice a big difference in ease of playing. In fact, doing this is a great way to warm up.  In my old band days I would play a bass for 15 minutes or so before grabbing my Les Paul and heading out to the gig.  

The answer is dependent upon the size of your fingers and hands.  In general, larger hands are more comfortable with a longer VSL and smaller hands are more comfortable with a  shorter VSL.  However, the space between strings can also be a factor.  If your fingers are long or fat/stubby, a longer VSL and a wider fingerboard and string spacing is more comfortable.  If your fingers are short or thin, the width of the fingerboard is less of a factor but you might find a shorter VSL more comfortable.

For the most part, assuming your hands and fingers are a good fit, switching back and forth with dulcimers having a difference in VSL does not usually cause a problem.  The left hand can usually adjust to the minor differences fairly easily.

Less sustain is not necessarily true. My 26" vsl models  tunes to dadd pitch two inches sooner than a 28" vsl so there is less tension on the strings allowing for sustain. There is a lot less chance for string breakage as well. ..Robert

Don, others have asked this question, too, assuming that the small differences in distances between frets would pose problems.  But I've never found it to be a problem.  I do think it helps that I tend to look at the fretboard rather than tablature when I play, but I think you'll get used to whatever instruments you play.  I regularly play instruments whose VSLs range from 17" on an octave dulcimer to 28-1/2" on one I string as a baritone.

As @Robert-Schuler explains, it is easier to finger certain chords on smaller dulcimers, which is why my main dulcimer now has a 25" VSL.  But there is also a trade-off, as shorter strings in general have less sustain.

Short scale dulcimers are much easier to finger when playing chords. I mostly build with 26" VSL. It's a happy medium...Robert

I don't see why it would.  The difference of distances between frets is pretty small.  So finger placement even with a 3" difference in VSL isn't  that critical, IMHO.  This table compares the distance between frets on 25" VSL with 28" VSL, and there seldom more than .2 difference in fret spacing between 25" and 28" VSLs.  

Scale Length: 25.00

Fret Dist. from Fret

1.    2.73

2.    2.43

3.    1.11

4.    2.04

5.    1.82

6.    0.83

7.    1.53

8.    1.36

9.    1.21

10.   0.56

11.   1.02

12.   0.91

13.   0.42

14.   0.77

15.   0.68

Scale Length: 28.00 

Fret Dist. from Fret

1.    3.05

2.    2.72

3.    1.25

4.    2.29

5.    2.04

6.    0.93

7.    1.71

8.    1.53

9.    1.36

10.  0.62

11.  1.14

12.  1.02

13.  0.47

14.  0.86

15.  0.76

Does playing different VSL impede the learning for a beginner?