svn trunk describes -f as:
-f Enable fullscreen mode. This overrides the window manager and causes the rdesktop window to fully cover the current screen. Fullscreen mode can be toggled at any time using Ctrl-Alt-Enter.
which I would have to describe as "not quite correct"/"misleading":
Support of fullscreen mode is "enabled" all the time (think "enable someone to have a feature activated"), it's simply that that support currently is not "active" (think "feature not active").
Thus improve the description of this cmdline option (and thus documentation of fullscreen support) to "Request fullscreen mode at startup time already." (git-svn diff attached). Or perhaps you happen to have more suitable wording suggestions :) (alternative wording, perhaps more elegant/terse: "Activate fullscreen mode at startup"; or "Request rdesktop to start up in fullscreen mode")
Thanks very much for a definitely very useful and stable piece of software!