Well, one size does not fit all when discussing clothing or penile size preferences.
Honestly, sex isn't all about the penis or vagina. Sex is much more involved and intricate than simply sticking it somewhere. *grin*
When partners feel secure in their relationship they talk to one another about what pleases them, and what they want. Do you believe your partner feels compfortable and secure with you?
Also, getting into a size discussion with your partner may be fruitless if she likes one thing and your thing ain't it. If she is willing to love you and be with you even if your member doesn't measure up -- or is off the charts -- what's the problem?
If you think that she is not enjoying sex, it may not have anything to do with the size of your penis.
Personally, I think that if you are a thoughtful lover, make your partner feel accepted, beautiful, and appreciated, spend more time on foreplay than on penetration, and put your partner first -- I don't think it will matter what size your member is.Cheers!