Ok, If you're big, don't ever ram it in, you can rip something, and will cause her not to enjoy sex, a close friend of mine had that happen to her, she still enjoys sex, but afterwards she has pain. My girl is always tight, I'm always having to prime, I normally start with one finger and work my way up to three, otherwise I will never fit, And it still takes some time for me to fit. You could always go down on her, get her off, get her used to one finger, then two, then three, Sex shouldn't be rushed in your case, it will take some time, and tell her it's going to hurt..... But don't let that discourage her, let her know the first few minutes will hurt, then she will enjoy it a lot more.
If that doesn't work, You may want to use some lube, there are many different kinds, But K-Y happens to be a favourite of mine, and it also enhances the pleasure for both you and your girl, When applying the lube, make sure you Lube her AND yourself for the best results.
Have fun
Payne