what is the max size wheels that can be fitted in the fender without rubbing issues

This is not the maximum size, but it gives you some perspective.

I was running an S3 8P 2007 with stock suspension and 19" wheels (tyre 235/35/19), an 8.5J rim and a 42mm offset I had rubbing at the rear with 2 passengers when suspension compressed on a large undulation in the road.

Supposedly if you remove the rear wheel arch top screws you can remedy this but I never bothered as it wasn't constant.

I'm sure I've seen someone on here running 20's, but have no idea what set up / measurements / ride heights they were running.

The lad who can ask all our questions isn't about at present, where are you N8?!