It's the same rims front and rear. In Scotland I put them on October through to April.
Not near the car so can't confirm the size, but we opted for 19" rims and winter tyres, rather than swapping out summer tyres every year onto the same rims.
The winter wheels are powder coated, which is just as well as diamond cut is a waste of time, with water, road salt, etc.
Lightly, and I mean lightly grease the wheel bolts. BWM locking wheel bolts are particularly prone to seizing. Don't over-torque them.
Remember and reset the air pressure monitor TPMS afterwards. Job done.
2015 i3 REx, 60ah, solar orange. Reversing camera, leather interior, sunroof, park assist, professional navigation.