Hi Larry (Dart),

By default, I don't have any hotkey associated with the full-screen mode toggle.
Larry (Wood) probably has assigned Ctrl-C to that function.

If you have the legend turned on, you can right-click on the constellation shown
in the legend (just to the right of the declination) to toggle full-screen mode.
Unlike the toolbar, the legend is shown even in full-screen mode.

In addition/alternatively, as with most actions in Guide, you can assign a
hotkey. Use Settings... Toolbar, and scroll down to where it says "full-screen
mode" (it's about 90% of the way down the list). You'll then see a "Choose Hotkey"
option (or that button will show the currently-assigned hotkey for that action).
Click on "Choose Hotkey", and (logically enough) choose the hotkey.

-- Bill