The originals are 6 inches from the edges of the plastic mounting. When you take out the standard door speakers the ring they are mounted to comes out with it as the speaker is attatched directly to it, so you need adaptor rings to mount new speakers on, you can get 6.5 inch adaptor rings that screw directly in. These go behind the door trim so you won't be showing off what speakers you have to thieves

Replacing the tweeter is easy enough, I think most tweeters will fit where the originals are (behind the small grille in front of the door handle) it's just a case of finding a way to mount them. Bare in mind that if you get a set of components you won't be able to use the standard tweeter wires unless the new tweeters have a capacitor attatched to them. Simple really, if they have a crossover box you won't use the standard tweeter wiring.
Hope I didn't babble too much lol.