I have a pool....just kidding. I don't want one, but my friend dose. The guy in the store said they max out at like 3' in aquariums. So how big of a tank would you need.
They get big but they are not that active. They only move when they eat or go up for air. That the reason I got rid of my 2ft electric eel. Seemed wrong to keep a tank for 1 fish that isn't that active. Although it would be cool to keep a few together in one tank. $150 is okay for pet store price. I paid around that for mine. A very hard fish to re-sell though.
Here is a video of my old eel:
http://www.youtube.com/watch?v=wfEyU5BoNcA