I use a 40W spot light in a space that size. It tends to produce more heat in one spot, but heat the overall space less.
Anything between 40-60W ceramic element should be about right for a 2ft tank. I don't know for sure, since most of my tanks are bigger. You should use a thermostat, especially with higher wattage bulbs. Keep an eye on the 'cool end' and try to keep it down around 25 C. Your python needs to be able to escape the heat too.