Lightroom CC is wider that the screen in Windows 10. Since adobe doesn't implement normal Windows controls, how do I return Lightroom CC to screen size?
It would be nice to see a screenshot to help us understand the problem. However, Shift+ F will toggle through the different screen modes. Does doing that make any difference for you?
Please go to the Preferences of Lightroom, then Interface Tab.
And please change the Font Size to Small, hit enter key.