slaveboy, you're really excited about this, and that's a really cool feeling I know...but don't let her see you're freaking out a bit, because if she's shy talking about it, this will make her retreat more. I know, I made this mistake myself.
If this is early days for you guys in talking about this subject, you need to ease her into it. Her natural reaction will be to avoid talking about something she's "not supposed to talk about with her partner". It's because she loves you and doesn't want to hurt you.
I can also tell you that yes, shorter and more petite girls are often also less deep inside, and a cock too big can be uncomfortable or even painful. My girlfriend is 5'7" and has had this problem.
In my case, I'm lucky enough to know that my girlfriend has been with over 30 guys before me, and that I am "slightly smaller than average" in her experience 
