In the global extension settings you can set the width of an opened card (the back of the card) to a specific percentage of the total board width.
If you enter a value a minimum width of 400 pixels will also be applied, if you leave this blank Trello’s standard card width is unchanged
This is a global setting that is active on all boards and it cannot be set per board.
If you enable this setting a minimum card width of 400 pixels will be applied.