Keyboard command/shortcut to resize window

Hold down Alt and press space.

Then press R on your keyboard. You can now use the arrows to resize the window.

You can press Alt + F8 and your mouse pointer will automatically switch to a resizing pointer, which you can use to resize your window either with the mouse or using the arrow keys.

You can then click or press esc to stop and turn your normal pointer back on.

I tend to use alt + middle-drag to resize windows.

It's not pure keyboard but it is useful in awkward situations (like this)