........when you can't play certain sections proficiently, no matter how much practice you put in!
I've just read this thread from beginning to end. Some people have posted the dimensions of their hands, so I thought I'd measure my left hand. If I've read the thread correctly, I think I have the smallest hands here, (of those who've declared their hand sizes).
Thumb to pinky = 18 cm
Index to pinky = 14 cm
Index = 6.8 cm
Middle = 7.2 cm
Ring = 6.4 cm
Pinky = 5 cm
Although I've played a 640 scale for many years, I've recently acquired a second 640, but with the added benefit of a string spacing of 42mm at the nut radiating to 50mm at the 12th fret to 58mm at the bridge. I find this makes quite a difference playing some chords on the lower frets. So, it's not just the string length that matters, but how close the strings are together.
"There is no duty we so much underrate as the duty of being happy.
By being happy we sow anonymous benefits upon the world."
Robert Louis Stevenson