If you're running an IB setup, mechanical power handling goes down. That being said, IB is not an SPL setup by any means, so power really shouldn't be of too much concern.
As for box size, that's up to you. I've seen 1:1 (vas:enclosure size) IBs and 600:1 IBs. The resonance will change as will the total Q of the system, so you're best bet is to model it and get the Q down to the point you want. Below .7 is considered to be a flat response design, above .7 is peakier (but most people like the sound of a higher than .7 total Q).