For all Joomla 3+ templates built using the Zen Grid Framework v4 (any theme after October 2014) please refer to the Zen Grid Framework v4 documentation.
The main content area in using grid template is controlled via the layout > main menu items in the template parameters. When you click on this item you'll see that there are a further seven sections of settings that give you control over how the main content and component space render on your page.
Each of these views are triggered according to combination of modules that you have published to the left, center, and right module positions. For instance if you are trying to determine the widths of the columns on a page that has only the left module position published then you would simply go to the Menu item that says to 2 Cols L as per the screenshot below.
