I was trying to figure out what size front speakers to add. Not from a wattage standpoint, but the overall package size. I started playing with some paper cutouts, then I decided to come up with a better drawing:
You can download a copy of it here:
http://transam.upiprime.com/forums/speaker_selection.pdfJust print it out full size, and cut out the size you want, to test for the fit. I am sure there are a few more sizes available, but this will give you a good idea of how much room particular speakers will take up.
Enjoy.