How big to plecostomus algae eaters get?

It depends not only on the type of algae eater, but also the size of the tank. If you're talking about plecostamus, they can grow to 6 feet long in the wild, and in a tank will usually die once they hit two feet long. (sad, really) All life has a genetic imperative to reach it's full size. If you put an animal (fish or whatever) in a container too small for it, it will become so stressed, eventually, that it will die. That is why people think goldfish live for only 2 or 3 years, when in actuality their life span is closer to 25 years. If you get a pleco, try to get a dwarf (7 inches long fully grown). If you are looking for an algae eater, try an otocinclus, a coryadora, or a mystery/Apple snail. They do a more thorough job and are less of a hassle to clean up after. Good luck!