[Nathan]

Start by figuring out how many air changes per hour you need, then with the size of the room you can get to how much flow in CFM you need. You can get a bathroom fan, wall prop fan or a duct fan all of them will work the same as long as they can hit the performance. If you buy a 50 CFM bathroom fan meant for an apartment bathroom you aren't going to get any flow (that is what my computer fan does) but if you buy a 1500 CFM bathroom or inline cabinet fan you will get a lot more.

Select the type of fan based on looks and fit then size the fan correctly for the job. A bathroom or cabinet fan is nice because they are square and are easy to fit in the joists, a round duct fan could be nice to mount on the ductwork but then you have to support it, a wallprop fan is nice to mount on the wall. The bathroom fan is also nice because you can get versions with internal backdraft dampers to help with air getting in when you don't want it to. Also centrifugal fans will be able to overcome the extra pressure if it is windy outside trying to blow air back in your ductwork when the fan is on. An axial fan is more likely to stall if the air is blowing back into your ductwork.