On my 335xi with staggered setup I went with 225/45/18 on the front and 255/40/18 on the rear for extra side wall. It made my speedo more accurate too! The diameter difference (front to back) is the same as stock.
See my post below.
2008 335xi Sedan, Sport, Premium [SIGPIC][/SIGPIC] IPod, Winter Pack, M-Tech Spoiler, V1