I find it hard to believe (no pun intended) that you are too big. The vagina is made to expand to accomodate the birth of an infant, and even if you are humongous, I am sure that you don't come close to the size of an infant. I think that you should look for other explanations. First of all check to make sure that she is wet enough when you try to penetrate her. If a woman is dry, even a needle size penis will have trouble entering.
You also want to check on whether she suffers from vaginisimus, which is an involuntary contraction of the muscles of the lower third of the vagina. Vaginisimus makes penetration (even with a finger) impossible. To find out if the is the case, ask her if she has ever had intercourse before. If this is an ongoing problem, then she needs to see a good sex therapist who can determine the cause of her vaginisimus. Most common causes of this condition are conflicts around intimacy and previous sexual abuse.
If she has been able to have intercourse prior to being with you, then you need to find out what has changed. Was she emotionally or physically damaged in her previous relationship? If so, she may be shutting out all physical intimacy in order to protect herself. Now is the time to engage in open discussions aimed at finding the cause or causes of the problem.
Once you are clear on the cause, 90% of the battle is won. Then, with the 'enemy ' in your sights, you can work on solving the problem that you have detected. I hope that you are dealing with a simple lack of lubrication. If this is so, then use a natural lubricant before trying to penetrate her next time. Also, keep in mind that lack of lubrication can be a sign of an undetected medical condition such as diabetes. So, have her see a doctor if dryness turns out to be the culprit. If she isn't dry, then you will need to assume that she is suffering from vaginisimus and get professional help. Good luck.