I thought that if you are running one of the recommended wheel/tyre size options in the 16 to 19 inch rim size range then the outside tyre diameter is basically the same so that they all 'fit' in the spare wheel well. The compromise is that the width of the larger sizes is higher than the boot floor which puts a lump in the floor cover that will eventually get distorted/ damaged when loads are put in on it.

My car is fitted with 225/45/17 tyres and the spare is a steel 205/55/16 which fits easily over my smaller (103tsi) front brakes.
It would not take much effort to build up the floor height with sufficient layers of offcut carpet or underlay for a larger or full size spare. You could use plywood but I'd resent the weight and lack of noise suppression that went with it.
The spare was adequate for a 250 km late night country run at a slightly compromised speed, My only complaint was how noisy it was.
You could run the 16 inch spare in the RS versions even with the bigger front brakes but that means that if the puncture was on the front wheel then you would have to put the spare on a rear and move the rear wheel forward. Two changes required but there is only a 50/50 chance of that.
I pity those that have more expensive German cars fitted with run-flats. Totally wrong for this country (imo) unless you never venture beyond metropolitan areas.