Everyone who's posted cannot give you any accurate advice unless they know how cold the room will get. A rule of thumb is 4 watts per gallon every 9 degrees below the temperature of the tank the room will get. If you want your tank at say 80 degrees F:
-500 watts if room stays around 75
-1000 watts if room stays around 70
-1500 watts if room stays around 65
-2000 watts if room stays around 60
That's a very basic guideline. Anything significantly less than those numbers and the heaters may be able to hold the tank at the temp you want, but they will be working harder, using more electricity, possibly decreasing the life of the heater, and you could also have problems if say the room drops in temp suddenly or something as it will take much longer to heat the tank up.
On my 225 I just bought a 1000 watt titanium, as my house is usually around 70 degrees.