since your going to be useing a thermostat I would go with a 250 watt or a good sized one. They can be easily underrated. I would go big and regulate. I have one in my boa cage that is 150 or 175 and it barely does the job on high and thats only 85 to 88 degress 1.5 feet up from floor.
:Before the debate on UTH vs Overhead heating begins, I prefer to use infrared directed on a ceramic tile or rock to provide the belly heat leo's so desire. Just habit developed from keeping iguanas and turtles to long who knows :) well minus the UV for the Leos thankfully