13cm or about 5.5" in old money, I believe.
T'aint a straight swap, btw. You may have to make some mounting plates up to go behind the door trim or cut the old cone out of its plastic mount and use that.
Word to the wise: don't go for whopper loud ones with massive magnets on the back or they will foul the window glass when it drops down. Especially on the back doors. You may have to mount them proud of the door trim using spacers in that case.