Haven't found the sizes either. I did come up with good documents on how to remove all the trims in the trunk for rear speakers. I am not sure if the big fixed piece on the side (wheel arch trim as BMW calls it) needs to be removed. I remeber that being a pain in E46. X3 seems a little easier to do.

To get to the rear speakers my thinking is that you do the following steps:
1. Remove upper D-pillar plastics
2. Remove C-pillar plastics
3. Remove the black plastic cover on rear seat belt (not sure if needed)
--> these are needed to be able to remove the key thing?
4. Remove window cavity cover strip being the plastic piece below trunk
side window that has the speaker cover

- p