I wear 7 1/2, and I'd be happier with a little bigger. The X1 is definitely at it's limit on my head unless I bend the leather band inward for a longer fit. Like Axel, my head is deeper than wider. I just have a big head in general though.
My biggest problem with hjeadphones is the cups not reaching my ears in the optimal position.
I.E. on Beyerdynamic Premiums (770/880/990), I basically wear them fully extended, and would appreciate a little more extension for a slightly looser fit. I don't like feeling of headphones when they're completely maxed out.
I wish more headphones were like the HD650. That has seemingly an infinite amount of extension. You'd have to be an absolute giant for the extension to run out. Watermelon heads. I know people complain about clamp, but I find the 650 very, very comfortable after stretching it out a bit over books or game cases, etc.