The door speakers are 4" with seperate tweeters. Not sure the diameter of the tweeters. 1.5" perhaps. There also is a woofer under each front seat. I think they are 8".

I just started the process of trying to upgrade the speakers in my '04 X3. Local Car Toys is steering me towards Focal's but their upper end models are pricy. (I also want to check out MB Quart and Car Toys does not carry them) I'm going to try to fit a 5.25" sepearate in the front stock locaiton and mount the tweeter either on the A pilar or on the black plastic piece at the front bottom of the door window. Plan to stick with 4" in the back. Not sure if the 5.25" will fit in the front even with modifying the mounting. The factory speaker location is very shallow too - only about 1.75". The factory speaker on the front door is a top mount and the magnet is just about pushing on the metal inside the door. Keep in mind, with a higher quality speaker, you may need a better amp to drive them to get the best sound.

HTH's...

Chip

PS...if anyone has been down the road of trying to fit a bigger speaker into the X3, I'd appreciate if you have any tips. Thanks.