[MyF15+F85]

Well, so it happens that I have one in my hand s I'm writing it... what to hell, I hope you need the front ones ( don't have the rear taken apart ) and I should say that this is of a 2010 335i front caliper ( donor car )
I hope that you don't want it like super exact, since I have my caliper at work and tape measure with me only, so hope this helps.
I can tell you that it is made out of some kind of aluminum or magnesium cast, since it is very light but has the grey metallic color to it. Made by ATE ( their logo stamped on the top of the piston and the numbers 5028 ).
Over all depth ( top to bottom outside ) 2" 3/16
inside the piston depth ( top to bottom ) 1" 7/8
Outside Dia 2- 5/16"
inside Dia 1- 13/16
groove for the seal 1/8"
depth of the groove 1/8"

Hope that helps