If you plan on having the store do it, they should have software/books that tell them the direct replacements. If you are gonna do it yourself, then they should be able to tell you what to use. Crutchfield may also be an option to tell you what you need. If i'm not mistaken, they usually run 4x6 in front and 6x9 in the rear.
www.crutchfield.com, go to "what fits my car"