It is not possible in Mac OS X itself, since Space does not actually activate a menu item, and the title of the menu item in the
File menu changes with the name of the selected object(s).
You could learn to press
Option-Space for fullscreen mode.
Alternatively, you can use hotkey/utility applications such as Butler or Keyboard Maestro for this, to intercept your key press and reroute it. The following steps are for Butler.
- Add a Smart Item » Keystrokes to Butler, and move it to the Hidden category.
- In the Keys tab, press
Cmd-Opt-Y (which is the regular Finder key combination for fullscreen/"slideshow" Quick Look.
- In the Triggers tab, press Space as hotkey, and confirm the dialog questioning your sanity.
- As Exceptions, select Only, and enter "Finder" in the text box.
A significant downside to this sledge-hammer-solution: You cannot ever press space in Finder to have it mean something else. Renaming folders, searching in Spotlight: Space is not going to work.