Why does Tor have a border?
It works by adding white margins to a browser window so that the window is as close as possible to the desired size while users are still in a couple of screen size buckets that prevent singling them out with the help of screen dimensions.
Why is Tor not full screen?
That concern is secondary and not a strong reason for keeping the window small, basically websites can see your screen resolution and if you're having a different resolution than other users they can narrow down there search results. Thats why it is not recommended to run tor in full screen.
How do I make Tor browser full screen?
ANSWER: On Windows, just right-click on Tor browser icon -> Properties -> Run -> Maximized. DONE.