All door speakers are 6.5". If you have the Bose system, the two front door speakers have their own amplifiers and the two rear speakers share a single amplifier in the driver's side rear trunk trim. The speakers are 2-ohm, and the amplifiers are proprietary. To rip out the Bose system, it is highly suggested to run your own speaker wiring to each door. It's also possible to bypass the amplifier wiring to retain the stock wiring, but it's easier and better to run new wire IMO. If you would like instructions on bypassing the amplifier wiring regardless, let me know.
The tweeters should be Clarion 4-ohm speakers and are really not that bad after you
do the tweeter mod.