Can't resize game window top/bottom
 

I must be missing something, how do I resize the game window top to bottom? I can make it narrower or wider just by dragging the sides, but I can't make it shorter doing the same thing. The best I can do when dragging from the large monitor to the smaller one is to maximize it, then at least I can see the top and bottom of the window.

Help! What am I doing wrong?