@Alex_755 says he is virginal. For his first time, length won't mean as much as girth.
If he's mainly interested in experiencing anal sex, find a considerate and understanding guy who has experience with virgins. Somebody you trust to go bare. (It goes in easier, finishes quicker, and gives you the full, natural experience.) I'd suggest a cock with moderate or somewhat larger head, and a slim shaft. Make sure he prepares you well - two or three fingers spreading plenty of lube INSIDE. Then onto his back - thighs open and legs up - like a girl gets fucked. Ease it in - it's gonna hurt - you may yelp, squeal or even cry as the head makes your muscle ring stretch and yield. It'll feel better when the anus closes around the slimmer shaft. A few smooth, easy strokes will lead to the very pleasurable sensations of being inseminated. The next time should go much easier.