In Dorico Pro, page sizes and paper sizes use different settings. This means that you can print layouts with any page size onto paper with a different paper size.
For each layout in your project, you can define a page size on the Page Setup page in Layout Options, which sets the dimensions of the layout.
When exporting layouts as graphics files, the layout’s page size is always used. When printing layouts, you must usually choose a paper size that is provided by the printer that you are using.
Normally, the layout's page size and the printed paper size match. However, if you define a layout with an unusual page size that is not supported by your printer, such as 10" x 13", one of the standard page sizes for instrumental parts, you may have to print the layout onto a different paper size. You can change the paper size in the Page Setup section of the Print Options panel according to your needs. As long as your printer supports sufficiently large paper for your page size settings, and they match a standard paper size, your dimensions are included in the menu. Changing the paper size has no effect on your layout's page size, and does not, therefore, affect the way the music is laid out.
If you do not select a specific paper size, Dorico Pro automatically chooses a paper size that is based on your computer's locale settings. For example, if these are set to a European country, an international ISO standard might be used, such as A4. If they are set to a North American country, one of their typical standards might be used, such as US Letter.
If you have defined a page size for your layout that is larger than a typical standard, Dorico Pro automatically chooses the next larger paper size, provided that your printer supports this. For example, if the layout's page size is larger than A4/US Letter, A3/Tabloid is used.
If you print to a different paper size than the layout's page size, Dorico Pro automatically scales the image to fit the paper. You can change this setting by specifying a custom scale factor in the Page Setup section.