I thought the size has to match what's supplied for OEM for any variation in the model.
i.e. a Golf 90TSI has 15" wheels, Golf 118TSI is 16", Golf GTI is 17" or 18" (optional), and Golf R is 18" or 19" (optional), so legally I could pick anywhere between 16-19.
The other stupid thing about ADRs is that since the Golf is treated as one model, the biggest wheels are the ones that must be at least 3kph/3% out (speedo compensation), which means my 16's are 8% out, and driving down the freeway doing 108kph is annoying when you have passengers in the back seat looking at the dash. I've connected to the onboard computer while driving and can see that the car knows the actual speed (although that's out by a more acceptable 2-3kph compared to GPS and advisory radar traps). You can only see this hidden speed by connecting a laptop or phone to the OBD2 port or by setting cruise control and then resetting the average speed data in the trip computer.