I would recommend a total wattage at 4-6 times the size of the tank depending on the maximum temperature dfference between where you want the tank to be and what is the actual room temperature. The higher the tank temperature has to be raised from actual rooom temperature the more wattage you will need.
That being said there can be other factors as well like whether the tank is up against a cold outside wall of the house, it gets a cold draft from an outside door, total water circulation to name a few.
I would recommend getting two smaller heaters that total your final wattage instrad of one large one for multiple reasons. If one gets stuck on it take alot longer before the water get too hot (heaters can wear out from cycling all the time and stick on). If you place one on each end of the tank you get better heat circulation (instead of most of the heat on one end). If one has to be removed or one breaks you still have one going in the tank for backup.
I admit this makes more sense as the tank get larger (20 or 30 gallons is pretty small) and some people do not like to look at multiple heaters. I agree that external in line heaters on multiple canisters work best (they are not even in the tank to view and are easy to adjust) but that is overkill for such a small system.