Windows

Windows lets you define custom page sizes.

You must first define the custom size in the Printer control panel. This is explained in this video (not created by GraphPad).

Once you create a custom page size in the Windows control panel, you can choose it in Prism's print dialog.

Mac

From Prism, drop the File menu and choose Page Setup. Then drop down the list of page sizes. At the bottom, find "Manage custom sizes".