I have 14" arms and do seated curls with decent form with 45lb dumbbells. I say decent cause I did notice myself cheating a little with my last 2 reps last time I did these.
This is also starting out fresh, I can't do the 45's with any kind of form after a good back workout.
So.. I'd have to say for the sake of argument that I curl 40's, cause I don't cheat at all at that.