There is a resize icon at the upper-right of the board. Click-hold on that and drag to the left. It can be a little hard to see on some of the backgrounds.
See the following topic, post #14 for an example, if my explanation doesn't help:
https://www.chess.com/forum/view/community/v3---tips-and-hints
How do you make the board smaller in the new version of the Chess.com UI? I don't see an option and Ctrl+- only shrinks the text not the board.