No, she wouldn't be so small that you couldn't get it in. She could be nervous and tight, especially if it's her first time. She might have been well lubricated, but still nervous and tight.

it is also a possibility that she has an intact hymen that is getting in the way. Do you feel anything when you finger her? Is she comfortable? Have you experimented with other kinds of sex before now? Would she be nervous about parents walking in/ getting pregnant, etc? All these could cause nervousness.

But babies can fit through there, a penis is definitely not a problem.

It is extremely unlikely that she is too small for you. A normal, average vagina can handle an above average penis just fine.

However, sometimes fitting in there can be difficult, painful, or just plain impossible if the girl is nervous or not lubricated enough. Sometimes people think the girl is lubricated enough until they buy store bought lubricant. Then they feel the difference and realize she might not have been as lubricated as they thought! So you can buy a water-based lubricant and see if it makes a difference. Being nervous can make the vagina clench shut. So make she is very relaxed before even attempting to have sex.

If this problem persists, she should talk to her ob-gyn about it.

i think the problem is more you than your girlfriend.

Do you know where you are sticking it in?
I know many 1st timers spend many times actually finding the right place to stick in.

Make her feel comfortable and tell her she has a beautiful body if shes nervous a lot of girls are very self conscious about their body. talk dirty to her and tease her and get her really wet. then take the head of your penis and spread her lube around the outside from the front to back making sure her hole vagina is wet. Its hard to get it in my g/f sometimes too. just make sure shes really wanting it. A lot of it has to do with foreplay to.. try rubbing her clit or G spot with your fingers first and get her loosened up a bit.