There is, just drag the border of the window... ;-)
feature request: quick chessboard resize options

Yes, good idea. I spend quite some time after logging in from a different machine changing the board size for Online Chess, Chess Mentor and Tactics Trainer, only have to change them again when I switch back to the original machine.
As an even more cunning option, use something cookie-based so that chess.com *remembers* these settings per-machine :-)
It would be nice if there was a way to resize the chessboard from inside a game (ie, without having to go into the settings menu). A small + and - sign somewhere in the interface would do the trick. I play chess from many different computers with a variety of screen sizes - so this would come in handy.
Keep up the great work!