I have had 18's, 19's, and 20's. Lots of folks running 18's which look best when running coil-overs. I personally like the track look of the 18's best. As far as sizing.... 10" in the front is the widest I would run and it will require significant negative camber (Nothing new to you judging by your picture). 12" is the widest I have seen anyone run in the rear but I wouldn't go wider than 11". I don't like the stretched look with tons of camber so I think 9.5" is perfect for the front.
'08 E90 M3 - Alpine White - Sold
'13 E92 M3 - Silverstone II - Sold
'04 E46 M3 - Jet Black - BBS E88, KW Clubsports, Brembo BBK, SuperSprint Exhaust, Recaro
'15 i3 - Laurel Gray - Why would I mod this thing?