6.5" front and rear (4-speakers). Front speakers are in the door, rears are in the rear deck. There are also little grills in the doors (near the door latch) where the optional OE tweeter kit would reside.
There is plenty of room in the rear deck for deep speakers (up till you run out of trunk space :lol: ) but you may run into issues in the doors with speakers that are too deep. I've got Infinity Kappa 63.3i (6.5" 3-way) speakers and the rears fit fine but the front required removing the rubber cap on the magnet or the window wouldn't be able to lower all the way.
I've got Sony Xplod's all around. I had no problems with fitment, I just got kinda pissed off when I took the little black covers off the door speakers and there was water inside. Glad I bought the warranty!