The amount of heat you generate doesn't directly affect the temperature you run at (as long as it is above the target temperature) - it more effects the speed at which you reach that temperature.
You need a feedback loop consisting of a temperature sensor which can report to the Arduino what the actual temperature is, and if it is below your target temperature then turn the heating element on. If it is above it, turn it off.
The more powerful your heating element, the quicker you will reach the target temperature, and the less time it will spend below your target temperature.
Anything that generates heat can be used. I don't know how big this cell incubator needs to be, so I can't recommend what is a good heat source. Yes, resistors are fine. There is simple maths that can be used to calculate the temperature they will run at.
The amount of power they will "consume" - V²/R, so if you run at 12V, and have 10? resistors, 12²/10 = 144/10 = 14.4W.