Yournameisrodney: Either 5x7s if you want oval speakers... or up to 6 1/2" round speakers depending on the size of the magnet. My 6 1/2"s have massive magnets and in my mates car they only needed a bit of wiggling around Also these put out plenty of bassy midrange sound so with a decent set of 6 1/2s and a small sub it would be mad... I would measure it for you but as you know i cut the holes...
hope this helps