>>>>>How do I change the size of the page setup to the correct one ?
On Mac you use the same command as on PC - PAGESETUP:
http://help.autodesk.com/view/ACDMAC/2020/ENU/?guid=GUID-5E728224-8273-4416-8BBA-CDBC87160F6F
Menu File -> Page Setup Manager.
Page Setup Manager dialog looks a bit different then in Windows version, but it makes the same job.
+ button will add new Page Setup
"gear" menu on the bottom of Page Setup list allows you to Edit and manage selected Page Setup.
You can change paper size in Page Setup dialog which you can open from Page Setup Manager.