I just put four 6 1/2" Pioneer TS-A1686R speakers in my 03 Tahoe (non Bose) on Sunday. The speakers that came out of the truck were the same size, front and rear. You can also use 5 1/4" if you want with the adapter that Crutchfield includes. There is no adapter needed for the 6 1/2". The factory front door speakers use an integral plastic bracket that clips the speaker into the door. The bracket can't be transferred, so it's necessary to drill new holes and mount the new speaker directly to the door. On the rear doors, the factory speakers screw into the doors (no adapters), so no drilling is required, 6 1/2"s are a direct replacement.
The Bose system also adds speakers to the A and C pillars, and an amp in the console, so you'd want to pull everything from a complete truck including wiring if you go that way.