In tanks 4 feet and over in length, it is best to have two heaters. If the filter has the intake at one end and the outflow at the other, which is best for circulation, a heater next to these works very well as the heater warms the water passing over it coming in to the tank, and the opther warms it going out to the filter so there is better consistency. Two heaters not only work better at heating the tank, they last longer because they are not individually doing as much "work."
As for size, 200w is good (thinking of two in the tank here). The higher wattage heaters are better constructed, at any rate they certainly last longer and do not malfunction nearly as much as 50w for instance.
If the above is not possible, an alternative is to lay the heater at an angle on the tank wall so the heat rises from a greater horizontal plane.
The heaters should remain in the tank year-round. In warm weather, if the room reaches or exceeds the set temperature of the heater, they simply won't come on. But you want them there in case it lowers at night. A fairly steady temperature is best. Some aquarists do have two sets on timers so one set comes on during the day to keep the tank at say 78F, while the other is only on a night and set to perhaps 75F. The idea here is to replicate the natural day/night fluctuation in nature. This is good thinking, although I personally don't bother, I have mine set at 78F (81F for the Asian with rare gourami) permanently.