Custom page size for printing?

I need to print to a custom page size, but can’t find how this is done.
On Dorico 3 I thought I was able. Any suggestions?

You can set the page size for a layout in Setup>Layout Options>Page Setup>Page Size. If you export the layout as e.g. PDF, this is the size that’s used. If you print directly from Dorico, the paper options available depend on the printer selected - for instance, you can’t tell Dorico to print to A3 if the selected printer can only do maximum A4.

You should also find that if you have set up a custom paper size in Page Setup elsewhere on your system, provided that matches one of the built-in paper sizes that Dorico knows about (even if it’s not one of the ones that appears in Layout Options), then that size will appear as an option, possibly under another name, in Print mode.