i havnt had any experience with unity and PC builds, however a lot of games in windows don't lock the mouse to the game windows, a good example is skyrim, instead (what I imagine they do) is to use the mouse movement (rather than position) to control the aim and make it so that when you click the mouse buttons it doesn't take you out of the windowed game and into windows.
this may be a better option as it would never limit you to the size of the screen (once the cursor goes fully to the edge of the window you wouldn't be able to move any more in that direction) and instead could get an infinite turn in your environment (first person shooter example)
unfortunately again I don't know how to make sure that your game window stays in focus when clicking outside of it, but the idea might give you a lead to search on.