Hi, thanks for writing in with your concerns. The average length of a vagina is 3-4 inches which can expand to 5-7 inches. The average length of a penis when flaccid/soft (not erect/hard) is 2-4 inches. The average length of a penis when erect/hard is 5-6 inches and the average thickness of a penis when erect is 3-4 inches around. It is quite common for there to be differences in size beween a vagina and penis but most will fit together comfortable for penetrative sex. It is also very common to be worried about sex hurting. You may find these resources helpful: www.scarleteen.com/article/advice/sex_hurts_her_but_i_dont_want_it_to
http://www.scarleteen.com/article/sexuality/first_intercourse_101
Let us know if this does not answer your question or if you have any more questions or concerns.
Health Nurse