What the OP is really talking about is whether or not a girl has a gag reflex. Two of my friend's wives have no gag reflex and can take me right to the base. Even though my SO has a gag reflex she still gives a great BJ. I've found that because I'm bigger than average in the girth department which the ladies love for vaginal sex but I find some women can't open their mouths wide enough to give me a good BJ without their teeth getting in the way. Like my wife says, as for length "anything over 6" is of no use." Rarely will you see a girl take all 8" in her pussy. I'm 6.5" and have hit many a cervix.
If you start off gently and work your way up, lots of women will take a larger than average cock. Nothing more erotic then seeing a cute little spinner look you in the eyes while taking it all the way in. (Lovely Laura?)