Fire and Tiretrack eels can reach 3 feet although I think 2 or so feet is more common in aquariums. The zig zag and peacock eels will reach between 7-10 inches. I have a zig zag eel in my 30 and he's doing great. Word of warning, they are jumpers and will find any opening you have. They are also not aggressive and can't really defend themselves but they will eat anything that moves and fits in their mouth.
They aren't actually eels but rather a really long fish, but when people refer to freshwater eels they usually come to mind because they are readily available.