Space saver is 18", at least the one in image above is.
Afaik the Golf came either with 16 or 18" space saver. GTD, GTI and R should have the latter. My GT had the 16", assuming that's because it came with 17" wheels.
I think it's mainly to do with what brakes it needs to fit over, remember you can't fit 16" rims to a PP/R.
Just a thought - if the rolling radius of the standard wheels on a GTI is the same as that of a base model (and bearing in mind how skinny a space saver is), how tall must the profile of tyre be on the base model 16" space saver. Think about a GTI (non PP) on 16's - now that's going to be a tall tyre. Then reduce the width of the rim by around a half. Now that's a high profile tyre!! I know the sidewall would be the same height, but the aspect ratio must be astronomical!
This of course assumes that they do 16" space savers and the rolling radius is the same.
As I said, just a thought...