If you really want to give her that experience during sex, you could try buying one of those Gorilla penis extender sleeve things that they sell online. But that's just a toy-type thing; if she still gives you pressure about your size, that's a huge red flag about her imo.
Big dicks are fun, but making a huge deal about dick size like that just makes it seem like she would be more likely to get with another guy just because he has a bigger cock.