Hi! Do you mean just a normal full screen mode for node webkit or "un-windowed" like a window app without the borders?
If you just want a normal fullscreen you can use the Browser Plugin actions. Just doubleclick on your layout, add Platform Specific "Browser" object and then you can use the following actions:
Request that the browser enter fullscreen mode. Note the browser may ignore this request unless the action is in a user-initiated event, such as a mouse click, key press, touch event or button press. There are three different fullscreen modes that can be entered:
Center - center the game viewport in the fullscreen view
Stretch (crop) - extend the game viewport to fill the fullscreen view
Stretch (scale) - scale (zoom) the game viewport to fill the fullscreen view
Return to windowed mode if the browser is currently in fullscreen mode.
This works in node-webkit exports. I use this for my new game.
Full details in the Construct Manual: https://www.scirra.com/manual/110/browser