It all depends on how much space you have.
Clarion makes a half-din center speaker but from what i've heard the Alpine is a little better.
If you have the room in your dash, you should put the same size speaker in as your left and right speakers. Otherwise you will need a processor that understands you don't have a full-range center channel. Usually they are capable of sending the center channel low frequencies to your left and right speakers instead. If you do have something like that then I would try to get a smaller speaker that best matches your front left and right speakers. For example, a 3.5" or 4" speaker from the same model line.
If you bought a component set, you could mount the driver further back in teh dash and port it to the front where the tweeter is mounted. I'm not sure how good it would sound but it may be worth a shot.