There are a lot of other reasons, but the first place you need to go, is your doctor for a physical exam to determine if there is a physical basis for the pain. Don't be worried about speaking frankly with the doctor, they have dealt with this before. They are interested in keeping you healthy in all parts of your life.
You probably have read a lot of literature already, but here are two more.
The American College of Obstetricians and Gynecologists put together a long pamphlet about the different causes and areas of pain, called Dyspareunia, experienced by many women.
The Mayo Clinic also did an article about Painful Intercourse. I doubt it is due solely to the length of your partner but it may be aggravating a problem. I hope you try for the exam and that the doctor can help.