For that you’ll want a full_width_row with spacers to lay things out the way you want. Alternatively, you’re right that those CSS rules are the place our “standard” width is defined, so you can certainly change them if you wish.
If you want to set a custom width for a card, you can put it in an XY Panel or a Flow Panel and either set the width in the editor or dynamically in your code. I haven’t tested with other containers but I know it works in an XY Panel and the Flow Panel.