I think it depends on whether or not you have the HK system installed. My car is a 2002 330Ci and it came with the HK system. It wasn't difficult for my installer to put 6.5" speakers in the door.
I believe the non-HK's are 5.25" (for both sedan and coupe). Again, I could be wrong since I have a coupe. I've had quite a few loaner 325/330i's and they all had 5.25" speakers in the doors.
Hopefully someone here who has the HK with the sedan can chime in.
Which system do you have - HK/Non-HK?