full screen mode in lightroom classic

If it is in really full screen, tap the green round button top left. Then it comes back to "full screen" with the top menu line visible, then tap twice on the top line in Lightroom. and it comes back to the last size you used.

Have you tried the Escape key?

Have you tried quitting the application?

Have you tried to Force Quit the application? (Cmd-Opt-Esc)

How about a reboot?

Did a little poking around over in Adobe forums and stumbled on this:

Solved: LR Classic on Mac Mojave stuck in full screen – Adobe Support

There's a little chatter about macOS full screen mode vs Lightroom's own full screen capability.

Maybe there's something in there to help.

Cmd-Shift-F?