I have seen them over a foot long many times (L. fasciatus anyway) And yes, they are aggressive, and do well in cichlid communities.
I currently have a L. fasciatus - teeny guy at 2.5 inches, a L. moralesi, about 4 inches, and an unknown Leporinus that nobody has been able to identify thus far. (Gray, with yellow throat, 6 inch range.)
They are known for being chasers and nippers. Nipping is much more destructive than, say, an aggressive Cichlid, because nipping simply does not stop, and it in fact eats away the other fish's fins on a continual basis.
The three I have are in a Cichlid setup and everybody fits in perfectly.
One very interesting observation though, my Midas (3 inches) is ALWAYS extremely aggressive towards the two larger Leporinus, but leave all the others alone - He must know they are nippers... The Leporinus simply flash away.
I have yet to see a Leporinus injured by anything else. They eat anything and everything, and even tore apart and ate one of my lily plant bulbs. They eat dead leaves off of the live plants, but leave the live ones alone, which is awesome. They have no end to their appetite, which makes for a spotless tank after feeding which is quite nice.