I believe 5.25" in the front and 4" in the rear... but PLEASE look it up somewhere on the net that can give you true sizing (maybe Cruchfield?).

I can tell you this... I would go with 6.5" front (if the magnet doesn't stick out too far in the back). I believe most who replace the fronts are going this route, and they definitely sound better. I went this way with my Infinity's a couple years ago and love them. I also cut the holes larger in the rear speaker brackets and installed 6.5's and they sound good as well.