How big a penis can a girl fit in her mouth?

I assume that would depend on the size of the girl's mouth. And how well she may or may not be able to control her gag reflex (the natural tendency to gag when something is placed at the back of the mouth and into the throat area).

Don't force anything that's not uncomfortable for you. You really can dislocate your jaw. If you do it once, it will be more prone to dislocate in future .