The front speakers are 6.5", rears are 6x9. With the front speakers, you'll find most are too deep to fit the plastic basket mounted in the door, but you can cut the basket out to allow the magnet of the new speakers to fit. The tweeters in the dash are just 3/8" mylar semi-dome tweeters... nothing special, but sound much better than a "dual cone" factory speaker by itself. Most aftermarket tweeters can be fit to the bracket on the back of the grille... which just pops out of the dash.

The car does have a double DIN opening, in that car, the deck should fill the entire opening. You can put a deck and another full DIN unit in your dash... Metra makes a Honda/Acura double DIN mounting kit to mount 2 Single DIN units in the dash.

If you really went crazy and got a double DIN headunit, there is an add-on piece that goes just below the radio to mount another single DIN unit... it is available from Honda, they were used when a customer got the optional cassette deck or 6-disc in dash changer.