How it works
The usual rule is about 5 watts per gallon to hold the water 10°F (5.5°C) above the room. A bigger gap needs more power, and a smaller gap needs less.
Use the coldest the room gets, such as a winter night with the heating off. A heater that is slightly too big just switches on less often. One that is too small never catches up.
Always pair a heater with a thermometer. Heaters fail, sometimes stuck on.