This is a very useful thread "kechance" thanks for posting.
In answer to your question my opinion is that one should play the smallest instrument that they are most comfortable with.
Take for example the Violin, it has a much shorter string length than any guitar, it has no frets but can be played buy virtuosos at incredible speed without much stretching, (Given the fact that chords are not played) other fretted instruments like the Mandolin, Bouzouki, Balalaika and even the Ukelele have shorter string lengths. I have seen these instruments played very fluently including chords, base and melodies.
Smaller finger movements make for greater speed and accuracy and if less stretching is needed then the hand is more relaxed.
I have always had a problem with playing some pieces because of string length. I have two guitars at the moment one has a string scale of 658mm the other is 650mm. The larger one has the more powerful sound, the smaller is a copy of a Torres but is easier to play, so I guess from the list above my hands would probably suit a scale of 630mm to 640mm. I am not worried about the guitars volume there are ways to get good volume and projection without long string lengths.
The new Guitar I am now starting to build will have a length of 640mm as a starting point to see if it becomes easier for my hands. Too small a string length will hinder some pieces due to the bunching of the fingers in the higher positions (8th and above) and too large a string length causes me to have to stretch too far.
Neck thickness is also very important a few guitars I have played in the past had thin necks and were hard to play when playing barres so for most part a Classical Guitar neck should not be too thin, lets say nothing under 23 mm, 25mm to 27mm would be more suitable.
Here are my left hand sizes for the database:
Thumb to pinky stretched 235mm
Pinky to index stretched 170mm
Pinky-Ring-Middle Stack 35mm
Pinky to Ring 90mm
Pinky to Middle 130mm
Width at the nut I prefer 50 to 52 with a string space spread of 45mm
Cheers Wayne S.