It has a nominal capacity of 50l, plus expansion tank plus filler neck. You could probably get 55 liters in there if you have enough patience and you're going to empty it fast again.
From my experience of driving VAG cars they have a fuel reserve. In my last 2 diesel Golfs I would get a warning 50 miles remaining. However, IIRC the fuel reserve was 8L and both cars did roughly 10 miles for every litre = 80miles on reserve. On several occasions I tested this theory and drove an extra 30miles after the car said 0 miles remaining. Though one time I did manage to put 57litres into my old 55L tank
I have never managed to put 50L into our A3 even when it said 0 miles remaining. The manual says it has a 7L reserve and the warning comes on at 50 miles remaining. It also does around 10 miles to a litre. So it should do another 20 miles past 0 miles remaining.
When you first get a warning on the Mk7 GTD does it say 35 miles remaining (like my GTI) or 50 miles remaining (like my previous MK6 GTD)?
I have twice managed to put nearly 53L into my GTI PP. The car had just said 0 miles remaining.
So either I have no extra fuel reserve past 0 and was running on fumes with a 50L tank. Or my car has a 55L tank the same as the Golf R has.
I would just have to drive 10 miles after the car said 0 remaining. Unfortunately, it is bad to allow a modern engine run out of fuel. So I'm not going to risk it with this car.