I have a tank almost exactly like yours... one gallon size. I was able to fit a Tetra 2-15 (50watt) heater in there... but I am not using the filtration, so I have a little more room...
The Tetra seems to be keeping my one gallon at 78F in a room with a low temp of 65F...but couldn't 'cut it' in the 5 gallon I bought it for! I couldn't reccomend this heater because although it's functioning (sort of), it's not adjustable.
I was looking at mini-heaters today and saw this:
. I have no direct experiance with Hydor, and I have not dealt with this online dealer... so, buyer beware!
I have bought Marineland Heaters and am very happy with the Stealth Pro shatter-proof '30' I got for my 5 gallon.
That link brings you to the marineland heater page. If you scroll down, you'll see they carry mini-heaters.
Finally, here is a selection of mini-heaters.
I would advise reading up on the specs of each heater, to find the one that will work the best for your
needs. Bear in mind that with adjustable heaters, you can compensate for wide room temperture/tank tempeture changes, and change the settings year round to minimize temp stress in your fish.
Good luck in your search!