See this topic from a couple of days ago:
The page size of your PDF’s should be created according to what you choose in Layout options, even if the panel in Print mode says something else. If that’s not happening for you, the solution to the topic above might be helpful. There is also a link in there to another topic where the slightly confusing UI in Graphic export is being discussed.
Ok, I went ahead and changed my printer destination to “Printer” Dorico then un-greyed the page setup drop down. I was then able to pick the correct page size and then switched back to graphics export. That works.
You don’t need to do that. Graphics export will always export to the page size you have defined in Layout Options. That’s why the page setup dropdown is disabled – it has no effect for Graphics export.
If you’re printing your Dorico project to a printer, then you can choose a different paper size from the page size you defined in Layout Options, and Dorico will scale the page size to fit the paper.
Good to know. Thank you Asherber! Maybe we can put that on the wish list for future newbies to have Dorico read your settings and display that in the Printer Dialog.