They take 13's as standard, however ive got 17cm ones fitted on custom baffles! A few people have said they look a bit silly, i think they look ok - but they sound awesome!
The hole on the door card is the same size as standard, just the baffle is drilled through the card and into the metal work inside the door. The speaker is then attached to the baffle. No other alterations are required, but its an incredibly tight fit. Any further one way and the speaker would hit the side of the dashboard when the door closes, and the other way it would hit the window winder handle.