from my experience most girls can only get a few inches in (I'm 6 and a bit in girth and 7 long) and I'd always found it difficult to come through oral..
despite a few who've tried, only one girl's managed to deep throat me.. most surprisingly of all she was skinny and petite, but certainley knew her way around a cock and on the whole took it as a challenge when I told her I'd never had DT.. as she did with most of our sex life, after failing at the first attempt and being vocal for a few days about her disappointment about not being able to fit it in.. visually it looked incredible esp as I thought it was never going to happen, but ultimately I'd rather attention was paid to the head and best of all my balls..
So to me it's more about technique, gag practice and willing than size of someones mouth.. god knows how I didn't cause her damage though!