The doors and the rear deck can take up to a 6.5" speaker. Stock speaker sizes for both the front and rear are 5.25".
The front door will actually accept up to an 8" speaker behind the facoty grill if you want to get that extreme. The rear deck will accept up to an 8" speaker as well. The important thing to pay attention to is mounting depth and how much clearance you have available. Remember in the doors you have to clear the power window, and on the rear deck you have to clear the slim black lift assist bars running under the rear deck to help open and close the trunk.