I THINK the diameter of the speakers is 4". The wire end will need to be cut and spade connectors attached to match the spades of the new speaker which is no big deal. Potentially, you could use a larger speaker provided you don't use a really deep one (outside wall interference) and the speakers aren't mounted in a curved area (won't sit flush).
I agree that Crutchfield is a quality place. I've bought from them for years without a problem.