Could buy your new plugs to make sure your size is correct. Then you might have to go and get a deep socket of that size and the extension will just fit(but not click)into the top of the socket if you hold it right.Or grind down the fat socket!
6.5inches. Most modern speakers won't match up perfectly on the holes. I've owned a 1993 Mazda protege and a 1991 Mazda protege, I've changed the head unit, speakers and added an amp and sub to both, so I know =p