If you use a nightly build then <hash>ToggleFullScreen</hash> will work.
If you use Dharma it depends on your keyboard layout because the hash key is in different places on different keyboards, and Dharma doesn't handle this especially elegantly. Your best option is to enable debug logging from System settings, System, Debugging, press the button on your remote and see what XBMC receives. On my system if I press # on my UK keyboard my log shows:
DEBUG: SDLKeyboard: scancode: 43, sym: 92, unicode: 35, modifier: 0
DEBUG: CApplication::OnKey: 61539 pressed, action is Number3
i.e. it thinks I've pressed the "3" key. I think this is because on the US keyboard (which Dharma tends to assume you're using) # is shift-3. You can add a mapping for the "3" key:
<three>ToggleFullScreen</three>
which is probably OK because you don't press "3" very often when using XBMC.
See
http://wiki.xbmc.org/index.php?title=Log_File for how to get at the log file. If you use Windows just press Windows-R and in the Run dialog type:
"%appdata%\xbmc\xbmc.log"
(including the quotes) and click OK.
JR