981 S alloy sizes?

[stewieyan]

Hi,
I'm a bit puzzled around alloy sized, can someone please help.
My Boxster 981 S came with "20 inch Carrera S" alloys and 235/35 265/35 ZR20 tyres.
I am thinking of 18 or 19 inch alloys.
What alloys/tyres can I replace this with, without jumping into Porsche non-approved territory?
Thanks

[stewieyan]

I was looking at the wrong section of the manual, cheers!