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A good candidate for the key to use by default is 'F11' to match the browsers. The keyboard shortcut should be configurable. What resolution should be used? A first step would use the same resolutions for windowed and fullscreen modes.
'F11' is fine as a default as long as this can be configured. Personally I'll switch it to 'f' (if available). The latter is a more 'linux-like' solution.
Same resolution for both modes sounds fine. The only issue I can think of is when using the max monitor resolution for the fullscreen mode: switching to windowed mode would make the window borders spill off the screen. But it's no biggy.