now you're asking...
potentially huge...are you willing to move the chassis rails? do you want tubs? what ride height are you runing at?
there are mk1s in Norway which I'm sure are running either 17 or 18s, (looks a bit weird though imo!) and I think Retro_al's one (check the resto section either here or on turbosport) can run 11" wide rears and 10" fronts, but that's with tubs and modifications to the bubbles.
potentially if you went for a drag style rear end and tubs that nearly met in the middle you could run really massive rims/tyres. depends on what you're using the car for i guess!

edited to add... i would say the typical on a tubbed car would be 10 or 11 by 15s on the rear and 9 or 10 by 15s on the front.