I'm about 8 1/2" long and 5 1/2" - 5 3/4" girth. When me and my wife first got together she told me we were not compatible because of my size. However, she told me it was the length not so much the width. In her words she said the length was too much but the width she could take a little more. Not sure her exact meaning, but to me it meant that I was very long but she could accomadate more in width if I had it to give. She always tells me she fills full duing sex, but has also said she couldn't feel it just to get me off she says. (Women are so confusing, lol)
Anyway, to answer your question 6" girth shouldnt be too much but will definately give her that stretched feeling that women like.