Honestly you will have to buy several, and just find which is right for you, I went to the dollar store and bought a 25W, a 40W, a 70W and a 100W. I ended up using the 100W, because although the 70W worked, it took a while to regain temp after opening the bator, the 100w gets back to the correct temp faster.
I made mine from a 40 qt rubbermade cooler, started with a 40 watt, which does work, but stayed on to long and made the temp flux to much. I went to a 52, and its perfect. cycles about a 3 minute interval, and the temp stays pretty stable. I use a drilled water heater thermostat and I think you need a bit more flux in temp to trip it. My fan blows right over the bulb and into the thermostat.
I made mine from a small Coleman cooler. I use a 40 wat bulb in mine. Oh and bit of advise don't buy cheap dollar store bulbs because they burn out quick. I bought some and they burned out three times while I had eggs in there. Here's a picture of mine.