app - can use custom paper size

Safari - NO
Firefox - YES
textedit - YES
Preview - NO
Mathmateca - YES
Excel - YES

p.s. The menu picks that lead to the dialog box seem to have a variety of choices. Some apps only had file/print, others had "file/page setup", or "file/page settings"

Not quite as uniform as I seem to remember it being under the old macOS.