I have a large penis. at 18 is it still growing?

Probably not?: At 18 you most likely have reached sexual maturity. Unless you have a particular hormonal issue, you should have just about reached where you are going to go. Bones may grow, but sexual organs should be about the size they are going to be.

Probably not: Provided you are wel through puberty and have stopped growing in hight.

Be happy with: what you have - as this is likely to be the extent of growth.