Your penis is probably not too big for her. A lot of men think their penis is too big, and a lot of women think their vagina is too small. That usually is not the case, however. A lot of people also think having a lot of sex will stretch the vagina out, but that isn't the case either. Having a baby can stretch the vagina, sex won't.
Your average female can take on a penis that is larger than yours even if it is her first time as long as she is in the mood and lubricated. Lack of lubrication is the number one cause of pain and bleeding during sexual activity. So buy a water-based lubricant and see if that makes a difference.
Since she is afraid of it hurting, she is probably tense. Being tense is the second most common cause of pain.
She should talk to her ob-gyn about this just to rule out other problems since pain and bleeding during sex can be a sign of bigger issues.