Concerns about a certain first time experience? Is all this worry for nothing?

Well I am 15 and about to be a sopre and I have a wonderful and gorgeous girl all to myself. We haven't really done anything more than kissing but I know it is time to take it to the next level. She has not so subtly "offered" to give me head, and she is actually really looking forward to it! (Weird right?) But any who I am really getting nervous. What if something goes wrong, and what if my isn't all she had hoped for. I have never tried to lead her on like I had a huge because in all reality it is only about 6 inches. I have read that it is an average size, but it just doesn't seem like it. Is it because I am 5'10? Will it look bad to her bc/ I don't really feel like it fits my body frame. Anyways should I be nervous? And what kind of hints will she give that lets me know when the time is right, because I don't want to seem like an wipe and for it to look like I am being selfish...