I asked her many times and she always starts looking a bit ashamed so I don't know what she thinks if it's to big or to small.
I assume you have looked closely at it yourself.
Depending upon the country you live in (born in or family religion) there should be percentage of men circumcised information you could Google. Similarly, there are statistics for penis size so you can figure where on the "bell-shaped curve you fall. Then you can look at it and see if it has stranger color combinations or unusual curvature.
So if you don't know, you should be able to guess at the likely reason, unless you are in self-denial.
Then armed with that, you should sit your GF down (not in the bedroom) and ask her if what you suspect is the reason is indeed the reason. I assume you know the difference.
See what she says, it might be totally something else, but then you will know. If she can't explain it to you, then explain to her, that while you would like to take your relationship to a closer more intimate level, you can't see that happening unless she can accept you and all your body parts as they are.
P.S. I am wondering if there might be a troll having some fun with us, but will play it serious for a while.