I'm 5.5" mid (5.75 base) and my gf would have a hard time blowing anything thicker. I think 6" is the ideal girth, at least that's the goal of most over in the PE section, and it was mine too until I started thinking about it.
Hehe, this might seem weird to some people but with different size toilet rolls ive tested fitting them in my mouth to simulate what it might be like for a girl taking my cock in her mouth. 5.5 inch girth toilet roll definitely fills up my mouth. A 5.75 inch girth toilet roll only just fits and causes some jaw tension, i think a 6 inch girth object would test my own jaw span and its probably going to be too much for most girls to take in their mouth for any significant length of time. Now if only i could test this out on my gf. "Ahh, sweety come over here for a minute i want to try fitting these toilet rolls in your mouth" I wonder how she would react hehe...
Just under 7 here and never had a problem.