It depends on what package you get. If you get the premium sound, you get the 12 speaker system and the speakers by the door handle are small tweeters (about 2''). they are the same as the tweeters beside the side mirors in the front. I cannot find the exact size in my owners manual. But I am sure if you Email Volvo or call a Volvo service department, they can give you the specs. Hope that helps some.
I called the dearlership here in Richardson, Texas and they dont have specs for speakers in the system. The guy up there said the speakers are sold as a set and the computer didnt give him any info on the individual sizes. Because I didnt want to take the whole door apart, I checked with Car Toys and Best Buy for the sizes. Both stores also could not look up the sizes but said that tweeters are universal speakers and that you can buy 1/2" all the way up to like 2" tweeters (not that sure of the exact sizes sold). The speakers would need to be fitted into the provided holes in the door. BUT...theres always a but to everything! Car Toys told me not to add speakers to the back without upgrading the amps because the factory amps will not be able to push out two more speakers and that the system could fry. Im not sure how much of this I believe. Im not a technician but I just find it hard to believe that two small tweeters would fry the system. Anybody have any input on this. Maybe someone in the forum has more experience with speaker systems.