X5 here in the US have 2 speakers per door. I replaced all of mine with Kicker 6.5" component speakers. Those woofers were too big to fit in the stock speaker holders so I trimmed it with snips. I'm guessing a 6" woofer would work best. The smaller speakers were replaced with tweeters. Front tweeters were held on by factory foam bracket and rears were held by hot glue. OEM rear tweeters were held in by glue also.

Keep in mind that the OEM amp has a built in crossover and rear speakers will not produce as much highs since BMW creates the front seat sound stage.

Look for BSW speaker upgrades for X5 in youtube. They show the inside of the door panels.

Heres is my upgrade post.
http://www.xoutpost.com/electronics/...tall-pics.html