I have no good experience with heaters I like in this size, unfortunately. I actually just went without a heater for the last few years on my nano's.
However, there is a Fluval heater that I am interested in trying. 25 watts. Google "Fluval Edge 25 watt heater". They aren't pricey, the are compact, and they attractive enough. They are preset however to 78 degrees so non adjustable. I plan on trying one when I have a few extra dollars.