First, I would stay away from american measurements. All 7/8ths are not the same. All 2 inch nuts are not the same. Metric is more accurate. A 650 mm (full size) will always be a 650mm.
Second, as with many things in life, size doesn't matter. It's what is comfortable for you and the sound that it produces.
Third, the neck shape is also a factor. "C" shape, "D" shape, etc. it can make a difference. My Rodriquez and Ramirez are the same size but the Ramirez has a slightly fatter neck.
I have 640mm with a 50mm nut. I had it for a couple of weeks and regretted not getting a 52mm nut.
If, you can try before you buy that would be great. If, you have access to someone' else's cg then try it. My wife (4'10") plays a 580mm with a 50mm nut. She also plays a 655mm x 50mm.
When, I first started with cg, I had problems making the stretch (especially getting my pinky to the F#) so I ordered a 640mm. Now I mostly play a 10 string. The scale is 650mm and the neck width is somewhere around 88mm.
2013 Jeff Medlin '37 Hauser 640mm sp
2006 Michele Della Giustina Concert 10 string 650mm ce
2005 Jose Ramirez 4E 650mm ce
2005 Manuel Rodriguez Model C3F 650mm sp
2003 Manuel Rodriguez Model D 650mm ce