two ways:
1. Visit http://support.chess.com/Knowledgebase/Article/View/25/3/how-do-i-change-my-board-size-or-pieces
2. hit control and then the mouse wheel to zoom in or out. if your mouse doesn't have a wheel, you can hit the + or - signs on the keyboard while holding down Control. :)
I have the size of the board larger than the size of my laptop's screen! I have sent a message to 'Help & support' at the bottom, but got no answer, if u know how to reduce the size of the board on 'Google Chrome' pls let me know.