speaker sizes and subwoofers that fit

I wish I had a selection of woofers on hand so I could see what's the biggest size I can fit behind my seat with it all the way back. I'm gonna get the measurements off a couple and make a cardboard template to see if it fits, but if someone has installed some... please save me some time. I don't see a 12" fitting back there, I hope at least I can fit a pair of 10's cause 8" would be weak!

I'm also curious about what ya'll have done to overcome the stock 4" speakers in your standard cabs.