The standard starting recommendation is something like 3-5W per gal
. Aside from this general guideline alot will depend on how warm you want to keep your tank and how cold the room temperature that the tank is placed will be. If you need to rase the tank more than 10* above room temp then you might need more heater than the recommended 3-5W per gal
I would start out with a 200W heater and see how that goes. You could also get a pair of 150's.