Currently, FreeMind has no function for showing and editing the mind map in a full screen mode, such one as you can find in Firefox or Adobe Acrobat Reader.
Missing in versions: 0.8.1, 0.9.0 RC series.
There is a patch that provides full screen mode, linked below. The patch is out of scope of FreeMind 0.9.0.
Keyboard shortcut for full screen: Firefox has F11; OpenOffice, including Writer and Calc, has Control + Shift + J; Adobe Acrobat Reader has Control + L.