Welcome to the forum. I am currently on 255/40/19s with stock suspension on a 2009 M. That size allows for a wider tire while maintaining the factory ride height and center of gravity for stability and safety. The grip and handling are definitely improved and the ride is still smooth without being floaty. Below is the chart found in the FAQ section of the forum. The chart will compare stock settings to any custom wheels and tire size - it also will provide differences between actual speed and indicated speed.
In the states, the OEM set up was 18x8 with 40 offset I believe and tires were 245/45/18.