I like to be able to catch all the instruments without them sounding melded together... like if there's a chime in there.. i wanna hear that chime real clean!
Log onto Crutchfield and put in your car info. It will tell you what sizes fit where. Then, just take it from there.
For a budget of $100, you will more than likely have a hard time finding speakers all the way around. One pair of good 6x9's can run you close to that, or more.