The size of a penis does not affect fertility, so yes, it's entirely possible for you to get your girlfriend pregnant regardless of your penis size. There are, however, a number of things that can affect the level of fertility in men, and I've outlined them below.
As with penis size, the size of a man's testicles has no bearing on whether he's fertile or not. What does make a massive difference, however, is the temperature of the testicles. Sperm can only be created and stored at a particular temperature, and increases in heat can reduce the level of the man's fertility.
Inner workings of the penis
As sperm is released from the testicles during intercourse, it travels through an opening inside the penis that works like a valve. If this opening is too small, only a limited amount of sperm can make it's way down the penis, and as such, the likelihood of conception is reduced.
The quality of sperm varies between men, but their job to fertilise the female's egg is a pretty tough one. The chances of conception are reduced significantly when a man only releases small amount of sperm each time. In addition to this, if the sperm aren't strong enough to make it to the egg, there is also little chance of conception.
If you're worried about your level of fertility, you should seek further advice from your doctor. There are a number of tests that can be performed, and a variety of medications available to increase the likelihood of conception.