^^^ That is a great response just check the wattage of your heater and compare that to the T-Stat. Keep in mind that most micro-heaters have 3 heat settings and each setting is a different wattage say 500W, 1000W, 1500W just as an example. So a 700W capable T-Stat could run that 1500W heat so long as it is only ever on it's lowest setting.
Personally though, I'd be more inclined to make a tub enclosure that will help insulate the tub itself and install heat tape on a T-stat, the dial on the micro heater should be sufficient to help stabilize the room temps so the heat tape doesn't have as much work to do.