You choose a Page Format, but say you want to further format the Content section with tables or divs or paragraphs, i.e. columns, of set width.  When the window size gets to a certain width, you are going to want these formatted sections to dynamically expand to 100% width, i.e. to be responsive.  You can do this from the Content Editor Box itself, no coding required!

1) Click on or highlight the content section in the Content Editor Box, and apply the "Expand 100%" Style to it:

It is under the Styles menu, and just scroll down to it, to highlight.  When it's selected, there will be a checkmark next to it, next time you open this menu and are on this content section that you applied this styling to.

You can also choose the styling "Expand 100% With Margin Top 10px" or "Expand 100% With Margin Top 20px", to add a top margin (spacing) of 10px or 20px respectively.

If you are not using tables or divs or setting widths to paragraphs to further format your content, and are just using the chosen Page Format, then the resizing of side columns, photo sections and text sections are done automatically already, based on your settings on the Styles Manager > Responsive App.