It depends on what motor you've got there Rich. They appear to fit one or two different types to drills, some are wound with heavy guage wire draw a lot of amps but are very high torque, others i have encountered are wound with thinner wire, draw less current but are low on torque. It's difficult to tell without seeing it but if you hold it in yor hand, connect it up and find it has a kick like a mule and spins up immediately, you probably have the higher torque version. Some of those types have an integral fan built into them to keep them cool because they need to dump the waste heat. Mankster is on the money with the gearbox suggestion though. If you just want simplicity and want to go direct drive, you dont really want a prop much bigger than 35-40mm, probably 3 blae types, otherwise the amps will go through the roof!
I have one of the heavy wound types in a rescue speedboat I built, came out of a drill just like yours. It drives a 35mm prop, I'm pumping 14.4 volts down it through two packs of 'c' cells and it's direct drive. It draws around 9Amps but goes like a rocket but of course the batteries peg out after about 10-15mins..
Chris