Originally Posted by crashcrf
I found this:
He's running a 265/30 up front and a 295/30 in the rear
Fronts look perfect, I would go with 275 rear, what should I go with 275-30-19? or 275-35-19
275/35R19 is the more common size to use on the M3 in general. But the larger height difference between 265/30R19 (25.2") and 275/35R19 (26.5") might be a little odd. Difference in height between the stock front and rear is 0.5".
That's part of why 265/30R19 and 295/30R19 is popular, it keeps the height difference similar to stock.
Given the sizes in the picture, his wheels are probably staggered.
275/30R19 square would probably be the best for what you are doing. With the 20mm offset on the wheels you propose, you could probably fine-tune the fit/stance a little with spacers to get exactly the positioning that you want within the wheel well as alofoque described doing.