Space saver for 2021 Allroad

[Jerbear]

Hi all. 
I have been through the search functions but still unsure as to what size space saver to get for my Allroad.

I see people have bought a 19” and got it to fit in the boot.When checking the rolling circumference of these they do not match the rolling circumference of my 245/45 R 18 wheels and tyres. I assume as a 4 wheel drive , this is more important. 
The size of the space saver is 125/70 x 19. 
Any help appreciated