They're 13cm speakers. The door cards come off if you take the screws out from behind the chrome door relese handles, and then there is a screw in the recess in the armrest bit (possibly hidden under a clip-on cover). Then there's the usual plastic clips but if you pull the bottom of the card away gently from the door, work your way around then lift it up it'll come off pretty easily.
The standard speakers have an odd-shaped mount with 3 screws in if I remember rightly, I made up some mounts from MDF but you can buy them off eBay pretty cheap.
http://www.ebay.co.uk/itm/Subaru-Imprez ... 19caf3ffbf