The vagina only really feels 3 inches deep.
If she has a cavernous vagina, you're going to have a bad time as it won't feel good for you, so she should be aware if it feels terrible for you too because of your size incompatibility issues.
There's always toys, foreplay and cunnilingus, but most importantly, ask your GF what she wants and be open about how you feel and what you are worried about.
