Type ALT+spacebar: this should pop up a menu at the upper left of the window that should look something like this:
Click on Size (or just type s): the cursor should change to a 4-pointed compass
Press the right arrow key (or the key that "points to" the window border that is off the screen): the cursor should jump to that window border - which of course you can't see right now, since it's off the screen.
Press the left arrow key (or the opposite of whatever key you pressed in the previous step) to resize the window: the edge of the window should move in the direction you've chosen.
Press Enter when done resizing.
Supported videos include:
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