WARNING: This page is about a different car, the 2007 BMW 550i, 2006 BMW 550i, 2005 BMW 545i, and 2004 BMW 545i. However, it is still accessible from the selected car via links, so may be relevant.
PARKING BRAKE SPECIFICATIONS - E60
Handbrake drum diameter
mm
185
Surface roughness of braking surfaces (precision-turned)