Data is from the Continental Technical data handbook 2008.
You can view and download it here:
125/70 CST ideal rim size is 3.5 +/- 0.5
135/70 CST ideal rim size is 4.0 +/- 0.5
No specific 145/70 CST tyre data but its looks like it has the same rim size limit as the 135/70.
IMO the important point to note in your review of space savers for the M3 is the unsuitability of the 125/70 tyre, not only because of the width issue but it also has a significantly lower load index/load capacity than the bigger section tyres.