Here's a really good answer for you, no it's not gonna cause her pain. Mine is the same size maybe just a millimeter bigger. The only issue that I've ever heard was one time when I had sex my first time with my virgin girlfriend after a few times she said that it was going too deep but it's not like rupture her internal insides kind of size that will damage her, no. There's a uterus up in there, and if anything is hitting it it's basically all it is is just a tap, and it's not like it's closed either, it has an opening and things can literally goe up into a woman's uterus no problem, I just really doubt that you're actually gonna cause your girlfriend any pain, I had sex with another girl later on and she was younger than me not older like the other girlfriend, but she still had no problems, but just so you know, it is easy to know whether your girlfriend is gonna be good for it, smaller girls have smaller everything, and the bigger your girlfriend is compared to other females, that should tell you what you need to know. Also the vagina extends when it's aroused.