The problem of size, ie. too big, depends entirely on who you are with. Some women prefer it big, some prefer it real big and some prefer it just ordinary to small. I never encountered anyone, who told me I was oversize, but then again I am just under 8" and almost 7" around, so I am not huge.
My brother however, is really substantial (11.5" long and coke can size around) and he tells me and I have no reason to doubt him, that the ladies who are size queens line up to try the merchandise.
I agree as well, that the chubby chicks are into big dicks, at least in my experience.
Key thing is stop feeling self conscious about it. If your girlfriend feels good about you, she will also be into your size as well. It really is mostly true that size is not a problem in general - unless you are really gigantic or really small. Most women like whatever they get unless they are out looking for something specific - which may happen with gay guys - but is not the case generally in my experience with the ladies.