I had no problems with the 10 upgrade but I cannot find a way to control the size of app windows. They're either minimized or maximized with nothing in between.
Solution: I had, somehow, set up Tablet mode (in Settings, System, Tablet Mode). Once I found that control and reset it everything went back to normal.
By the way - I've found Classic Shell to be a worthwhile utility for enhancing Windows 8 and 10.