I've had 19", 20", 22" all have their own reasons... 19" make the car sit super low to the ground with the right suspension but will scrape a lot!! 20" give it the perfect stance, fill the fender pretty nice and look natural on the M!! 22" would be great on a daily... They full the wheel well perfect!! They sit up high enough to clear speed bumps, parking stops, and most driveways... They do not make the M look 4x4 so that's a big plus!! Since you're are AWD, I would stick with a square set up but many have gone staggered!!

good luck and enjoy!!