how to open mail in full screen

control command F is the keyboard shortcut.

Like any other app, you can alternatively click the green pill; button.

Holding the option key and cicking the green pi;; button will expand the window to take up the entirety of the screen only.

Use apps in full screen on Mac - Apple Support

You can leave your Mail in full screen mode.

Use Mission Control on your Mac - Apple Support

with the preview screen in the typical 3-column view, there is no advantage in opening "individual emails in full screen—this would show mostly white space.

Are you having some issue here?