I like my eels pretty big, not huge congas, but big. I guess 18" is about right. I'm not crazy about smaller eels, not that they won't catch as well, but they are much more fiesty and tend to do that hated eel knot thing all the way up your %$#@ line, plus the larger ones cast better and seem to swim to the bottom quicker. Just make sure you rod can handle the weight of a larger eel, nothing worse than a graphite rod going off like a bullet in your hand when you try to lob a conga..
