What is the maximum size spare wheel will fit into spare wheel well?on a 2021 vrs 4x4 estate car.one other point,if I got a full size spare wheel ie 19 inch how could I make the floor level without it moving around.

I can only tell you that on my 67 plate Superb the full size was a 16" wheel with big balloon conti tire , not a space saver. If you ever fit one your ACC wont work while your on the spare.