My girlfriend is very petite and I'm reasonably well endowed so had similar issues starting out. It didn't help that I'm the first person she's dated or had sex with, so in addition to starting slow there was aspects of nerves for her in play.
I spent nearly a month slowly starting with massaging her to help her relax initially, eventually moving to teasing her clit, and then moving to going down on her to start. I then progressed after a time to using a single finger, eventually moving adding another finger after about a week with one. When we got to the point she could take 3 fingers and enjoy the experience we moved to her trying to take me PIV. Had some issues with her vagina clamping from nerves at this point so took another two weeks and talking over her concerns, nerves, etc before we eventually succeeded; however, now we have no issues with me getting in with minimal foreplay and a bit of lube. We need the lube more due to she doesn't get as wet now that she's on the pill, as that's seemingly a known side-effect.
But take it slow, use lube, relax, and most importantly you should both enjoy the process.