How to: Fixed width layout in 7.x-6.x ?

Is there a setting in Zen 7.x-6.x to state if a theme should be fixed or fluid width? Do you just set it manually with max-width, or does it need to use some zen grid function or something similar to do it properly? In older versions of Zen, I'd go into sass/layouts/ and modify the _fixed or _responsive scss files. Thank you!