simple, open explorer, maximise the window, then drag it to the desired size. Then close it using the file and close option NOT with the cross top right. Next time you open the window it should be full size. You could also try hitting F11 to make full screen and F11 again to undo
In future, I will recommend tea and biccies as a solution to all problems. Got a suggestion off a colleague to disable "always on top" & "Auto Hide" in the taskbar properties. Didn't fix it, but when I re-enabled them, ta da! Off for some more biccies and a discussion on the mysteries of MS software.