When you launch FreeOrion, it starts up in a teeny tiny window. I immediately clicked Options to look for a "make it go full screen" button, or better yet, an "always start in full screen" checkbox. Instead what I found was a pair of pop up menus which offer resolutions for full screen and windowed modes, followed by the unhelpful text "restart in full screen to take effect".
Now, you may not have noticed this, but the user is not presented with any way to choose whether to run in windowed or in full screen when they double click on the application icon! Quitting and relaunching the app after changing the fullscreen resolution pop up value does not help. It still launches windowed.
Firstly, the game should run in full screen by default.
Secondly, there should be a checkbox in the Video Options to switch to windowed mode for future launches.
Thirdly, there should be a menu item, button and command key available to toggle between windowed and fullscreen mode. I humbly suggest ⌘F.