You can use any size motors in any combination you like, as long as they will work properly with the drives you'll be using. The only thing you might want to consider is the voltage rating of the motors. If the voltage rating is very different, the lower voltage rated motors will perform a lot better if you're using the same power supply for all the motors (which most people do).
I'd use the most powerful motors where you needed the most power. Wherever you need to move the heaviest load, or the axis you want to move the fastest.