If this is a concern for her, she might try masturbating with various sized vibrators/dildos to help her get accustomed to a larger size.

Also, different positions can help. Try her on her back with her knees to her chest; sometimes that can help lengthen and slightly balloon the vaginal canal.

Foreplay and lots of it. Even if she's "wet" she may still be mentally unprepared. There are times with my husband where my body is rearing to go but I'm not, while my loins may respond, if I'm not in the mood than I'm not in the mood.

Foreplay can help open her body up and prepare her for sex. Watching porn together with included touching is a great way to relax before the big game. Taking things slowly and using lots of lubrication can work too. Make sure she's always comfortable. Try different positions, spooning is a favorite of mine because it allows my husband and I to remain close but it's also really relaxing.

Just take your time and enjoy yourselves in the process.

Heck! It's uncomfortable for me if I haven't been "warmed" properly! I totally agree with the previous posts! I also highly recommend a clitoral stimulator and lube. You should go slowly at first too just to get the juices flowing better. It's something that shouldn't be rushed.

LUBE LUBE LUBE! There is NOTHING wrong with using lubes to help with these sorts of situations. It doesn't mean someone is a failure in the bedroom, or that they don't get excited enough from their lover. Women go thru hormonal changes every single month, that can mess with their ability to self-lubricate. So can any medications (including birth control methods) that the woman may be taking. Get some good, quality water-based lube (safe for all toys and condoms) to have on hand. The more wet a woman is, the better the experience for both!

Don't feel bad, I have had a baby and I am still uncomfortable some times. However I have noticed that lube can take care of this. I have a hard time getting relaxed, maybe give her a massage with some nice oil and see where it leads. I have also fould that doggy style gives me more control over depth of penitration and is more comfortable in general.

Lubrication. I can't stress this enough.

Because of a lovely medical history, my vagina tends to be uncooperative. I'll be ready, and it's... well... not. Make sure you reassure her that you want her to feel comfortable. That was my biggest problem with new guys, I felt broken when I went to grab my lube and some of them made me feel like I was.
If it's really a problem, it helps when my boyfriend starts penetration extremely slowly. I know it's sometimes agonizing for him to have to go so slow, but it really helps me get ready and also to have the lubricant get everywhere it needs to be. Good luck!!

Lubrication and LOTS of foreplay.