Get a Hydor Theo 25 watt. You can order them off Amazon or Dr. Foster's and Smith, and they run something like $15. My 25 watt works fine in a 1 gallon and fine in a 3 gallon. I have a 50 watt in the 3 gallon now, and put the 25 watt in a 1.75 gallon.
The Theo heaters are adjustable and very small (about 7" long), reliable, and easy to use. The glass is supposedly shatterproof, and they supposedly switch themselves off if you accidentally take them out of the water without unplugging them (never tested that out).