Are you talking about a screen on a phone? How big is your screen? 6 cm x 6 cm is pretty good for a phone, since there have to be some controls outside the board.
Chess Board Is Too Small

You can go into your settings and change the size (on v2) Are you on the simple board?
This may also help.
https://support.chess.com/customer/en/portal/articles/1444786-how-do-i-change-my-board-size-or-pieces-
If you are in v3, hover over the upper right of the board, click and hold the resize icon, and drag to the right.
Hi, the chessboard is awfully small when you resize the windows. It was posted about 5 years ago, and someone said they were fixing this, but 5 years on, and it is still not fixed. Please fix this. My chessboard is about 6x6cm on my screen and there's no way to make it bigger...