The BMW 1 Series comes on 17" or 18" factory wheels. The most important improvement you can make is ditch the Run Flat tires for a set of performance tires. This simple modification will improve handeling and comfort. The 1 Series has no spare tire as standard this makes a tire-repair kit adviseable when upgrading to performance tires without the Run Flat feature. The weight of tires can vary by as much as 3 lbs. depending on the model so take note of this when you shop around.
19" aftermarket wheels on the 1 Series require a lower profile tire. The disadvantage of bigger wheels is that they weigh more. Bigger and heavier wheels increase unsprung wheight which decreases comfort, handeling and acceleration. The lower profile helps with steering response and the bigger wheels provide space for bigger brakes.
