The string length you suggest is still in the realm of a full size instrument ableit on the short end. Once you get to 560mm and less the the terz guitar (3/4 size) pitch of G and dedicated strings come into play. Most 19th century (early romantic) guitars have a string length of 610 - 630mm (some shorter like one Stauffer model at 597mm) and are tuned to A430-435 but will do fine at modern A440 pitch with appropriate tension strings. They don't have the same sustain as modern instruments and might not be as loud but with modern design and building techniques this will be less obvious. On the positive side the left hand stretches that stumped you before will be a breeze...gotta love it! You shouldn't have issues with intonation as long as your luthier has some experience with shorter string length instruments. The shorter the string length the longer the compensation needed to ensure good intonation. For 620mm the necessary compensation is in the area of 2mm+/-.
Here is a list of strings that I have found work well with shorter string length guitars. It's better to use strings designed for shorter string lengths as often ones designed for 650mm+ can feel soft and sound "thumpy" (for lack of a better word) lacking sustain and a clean tone.
Most of these can be purchased through [ Moderator edit: Direct links to commercial businesses are not allowed. Please do a search for Just Strings]
or from the manufacturer or distributor.
La Bella Strings- http://www.labella.com/
in Europe and http://www.aquilausa.com/
in North America
A. La Bella ERG#7 - Early Romantic Guitar Varnished Gut
B. La Bella ERG#1 - Early Romantic Guitar Nylon
C. La Bella RTG#1 - Early Romantic Guitar Nylon – Terz
D. Savarez 520 B White Traditional Classical Nylon Guitar Strings Low Tension
Work well on Terz or full size guitars
And a few (new to me) selections that have been brought to my attention by the kind folks at Savarez.
E. Savarez Prodige 58 (58-64cm) 540 CS
F. Savarez Prodige Alliance 58 (58-64cm) 540 AS
G. Savarez Prodige 38 (38-55cm) 540 CXS – Excellent Terz guitar strings
H. Aguila Ambra 800 (soon to have a name change, I'm told)
Please note, I don't get any kickback or benefit from any of these suppliers. This is just a copy and paste from some material I send out with my instruments.
To answer your real question, I find 620mm to be a very nice string length. Not too tight and doesn't feel much different than 650mm.