Well, in the famous words of mods and admins etc.
This is a forum dedicated to carpc-car audio integration, not car audio. Read the sticky above for links to specified sites. Someone here should be able to help if you're willing to stick around.
Crutchfield sez: 6.5's all around (but with brackets you can fit smaller ones, or 6 3/4, if you want to)
As for hooking them up, as long as you're not going to re-wire them it's easy
Disconnect old speakers' + and - leads (should be a phillips screw on each terminal)
*Gently* pry up the shroud on your rear deck
Use a power drill (or screw driver if you don't have a drill) to unscrew the mounting screws. You might have screws in the trunk instead of on the deck, so check in the trunk when you're disconnecting the leads.
Pop the new one in, connect the leads and you should be good to go.
If it's coaxial, i think you can pigtail connections on, but i don't know, never done that before
usually suck-it-city or worstbuy will do installs, but i know it's not cheap, it's $45 a pair, $95 if you want them to re-wire the car.
If you're adventurous i'd run new wires and all, then again, i'd also replace the fronts, add some tweeters, and shove a PC in there for good mesure too
someone else should be able to help you with coaxials or clarify what I said.