dpgx5 provides some good information on space saver wheels.

In short, if your car was 3.0 (gas or diesel) or 4.4 and was ordered with the space saver wheel, you would have the 18" version: BMW 36 11 6 750 316

The 4.6 & 4.8 with the larger front brakes (which take a mininum 18" NORMAL wheel size) would have the 19" space saver version: BMW 36 11 6 756 858

If you upgrade to larger brakes which requires a wheel (size) upgrade from 17", remember to check what you have for a spare tire. Usually a take-off 18" or 19" normal wheel is a better value alternative, unless you really want to save a few kg or lb in the trunk.