I'm looking to do the same thing, I have the same car, and I'm pretty sure they are 6 1/2" speakers in the front and middle, with 6x9"s in the cargo area...maybe that can help a little
It would be nice to find out the full specs for the stock HU so you could find speakers that matched the recommended RMS, so if you know where to find that I could help you find speakers that would fit and have the right power without replacing the HU