Our current offsets on our E90 M3 are pretty aggressive in terms of outer fender clearance
It's hard to tell in the photo, but this sizing is very close to the outer limit of what's possible on the stock sheetmetal on an E90 M3. The car currently has 20x9 +18 front, and 20x10.5 +18 rear wheels, with 255 and 285 tires in the front and rear, respectively. This setup is very aggressive, and will rub the fenders just a bit.
To maximize the amount of tire you can fit under the stock sheetmetal, I'd recommend running a higher offset than this- you'll be able to run beefier tires as a result. I'm not 100% on the inboard clearance on the E90 M3, but can measure this week. Going up in width and raising the offset will allow for you to run as much wheel and tire as possible.
I know this isn't exactly the answer you needed, but I hope it at least helps you see how much room you have on the outside of the fender. Once we figure out inboard clearance, you should have a pretty good idea of the maximum sizing you can run.