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20" would be the most I'd go. I'd recommend something like 245-40-20 F and 275-35-20 R with a really good set of tires (Michelin PSS or PS4S). That's stock on my M760i and it rides quite well. You could bump out the fitment with a 11 or 12 mm spacer if you want a more flush fit. You could also add width to both axles, but I think 295 is max on the rear without spacers or a different wheel offset. Not too sure on that though.
FWIW, The oem 21" options actually have the same width as the 20" but reduce sidewall width to 35 F and 30 R which would not be good for rough roads and ride comfort.
Alpina owners with the 21" tend to bend their rims quite often too so something else to consider. If you don't have a V12 or V8, you may have better luck not bending larger rims but Personally, I think the G series looks perfect with 20" if you have the right wheel style. But I also have a pre LCI which doesn't have as "big" styling as the LCI does so your miles may vary.
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