I played 640mm for more than 2 years. Once my fingers got used to it, every time I picked outmy flamenco guitar, 650mm, I felt like it was a very big instrument. I sold my 640mm eventually since not only itdid not improve my playing but I got a bit of unwanted tension in my left hand. The reason for that was thatmy 640mm got string spacing at nut a bit too narrow for me (I like 43.5mm). My hand span is around 8".My lesson is that the most important parameters in a guitar are string spacing (at nut and saddle)and neck thicnkess (I like very thin). Try to optimize these things first.Finally, if my hand span was 7" I would definitely go with 640mm or even 630mm.