The sticker in the driver's door should give you the correct tyre size - mine was T145/65 R60. I then matched all this info together and found a genuine Audi space saver online which fitted perfectly and housed the Subwoofer OK as well.
You'll want to ensure you have the right jack as well. I had originally ordered a road hero spare wheel kit but returned it because the wheel needed an adapter ring to fit the car, it wouldn't fit the subwoofer, and the jack was decidedly not safe looking in terms of connecting to the mounting point compared to the Audi specific jack. It looked like a good option but in the end I was only happy with what would have come from Audi.
It's imperative that you check and confirm all the details for your vehicle separately as there are so many variances!