You mentioned biggest so I'm assuming you want loud?
My bad if you already know this... You'll need an amp to power the speakers if you want loud because the stock HU blows balls. You can get mp3 + ipod connection. Alpine makes tons of different HUs, and you can ask broadfield about what you need to fit price range.
Screw 4x6, 6x9 and anything else. Get 6.5" speakers & adapters. You just have to mount speakers to adapter, then adapter to door panel. Either Broadfield ($5 w/ speaker purchase.) or ~$30 shipped on ebay (front and rear)
http://cgi.ebay.com/ebaymotors...60393h ... rs...09967
There are a **** load of speaker manufacturers... Focal, Infinity, Diamond, Sony, MA, Rockford Fosgate and countless others with low/med/high end speakers. I've heard good, the bad and everything in between, but one company sticks out, Boston Acoustics. check em out They have the baller Pro 60 and not so baller but still very good SL60.
I really don't know how much your trying to spend... If you feeling baller get 6.5s in the rear w/ adapters, but if your on a budget Boston S75 ($75 4x6) will do fine. It'll fit in stock location with no adapter.
In the middle of a sounds project myself... Learning everything as I go along... so if you have any questions, I've done a lot of the leg work so I could probably help you out a bit. Let me know bro..