6x8 is the stock speaker size. 6x9's can fit depending on models. Just search through Crutchfield or on eBay for a brand your familiar with. Any 6x8 should work. You can also use the search function on these forums for past threads, theres a new one about every week!!
This is the search...
http://contour.org/cgi/ultimatebb.cgi?ubb=search&search_forum=5
