This is the closest I could find while being OEM. Honestly, 20s are going to look ghetto. Max 19" on the E9X M3.
It looks like the stock wheels on the new M5 are 19" versions of our 18" stockers.
"Star Spoke (Style 345M) 19 x 9.0 front, 19 x 10.0 rear forged alloy wheels, and 265/40 front, 295/35 rear performance tires"
Wheels specs:
19" style 345M
Front 9JX19 ET32
Rear 10JX19 ET34
Visual Comparison:
ZCP Wheel size for comparison:
Style 359M
Front: 19 x 9 inch, ET31
Rear: 19 x 10 inch, ET25