The stock size is 5x7 or 6x8, both will work.
The plastic grille is part of the door panel.
If you plan to get an amp, you can use the stock size, or upgrade to round 6.5" component speakers. There's a How-To in the FAQ to build baffles.
I would stay away from Audioblah.
DRZ9255 ~ Arc Audio 4150XXK ~ Oz Audio 180CS ~ IDQ12 ~ iPod ~ Sirius