LOL any wattage will be fine, because it shuts off at the temp u set it to. to be safe, do not set it near the max of the temp range the fish live in, in case it heats up too quickly and goes over a bit.
I got a 50 watt for a 2 gallon, because I plan on getting my mom a 5 or 10 gallon tank for Christmas, so I want to transfer over that heater. =D It keeps that temp dead on 26! hehe.
Bought her a couple neon tetras. And they only have to live in there for 2 months, then they are going to the new one! So once they are all grown up! hehe