How wide should I make my game-room if I want to fit a "full-size" pool table in it?
You need enough room to be able to properly line up and execute any shot. I would say you need at least 5 to 6ft "elbow room" around the table.


  A regulation sized pool table is 4.5'x9'.


Source

So your room needs to be 14.5' x 19' to 16.5' x 21' (which sounds very large).