I just measured 86mm across the oil filter housing cover.
Probably simpler to just buy one specifically labelled for the car though, as I don't know if the number of flats is common. A band style would be tough to get purchase on, and of could there no way to properly torque the thing with one of those.
What size socket you'll need for the wrench will depend on which wrench you have. I think the aftermarket one I saw is a different socket size than the OEM BMW.