What Polifemo said... and a bit more. It all depends on the guitar, and what you are comfortable with.
Measured hand span has little to do with whether you need a longer or shorter scale: too many factors involved. My span, measured, is 8" -- I played a Ramirez 1a for years, and like he said, it's a tough play; it's the guitar's setup as much as anything that makes it a beast. Wide, dead-flat fingerboard, high action... but my hand, even after years of playing, while very flexible, is still relatively small. It was, in my case, getting in the way; I spent more energy trying to just play the darn thing, even with very good technique. I did throw in the towel and while I still have my beloved 1a, opted for a 640 from another luthier. And it *is* easier to play.
Does it solve every technical problem? Far from it; but it helps a lot. I've heard, anecdotally, that many players, even with bigger hands, are for shorter-scale instruments (ca 640) -- it's just easier on the LH. And why not? Playing the guitar shouldn't be a test of machismo!
I play guitar with classical mandolin groups. A tiny instrument like that (to my guitarist's eyes, anyone)... and yet one very fine player mentioned to me one day that she needed a shorter-scale mandolin. Now, I think the neck of the instrument she was playing was ca 16" nut to saddle...!