If I use a Porter Cable 3-3/4 router motor what size stepper motors do I need?

You can use the standard electronics combo: http://www.buildyourcnc.com/item/electronicsAndMotors-3axis-425-elcombo. The size of the stepping motors are generally sized with the overall machine structural weight, inertia of parts, the type of mechanical parts used to move the axes, and actually, less by the router which is more connect to how the machine is controlled.