How to change paragraph width in full screen

Hi guys,

First let me say thanks for a great customizable theme. I love what I can do with this theme.

Second, I am trying to make an update to the paragraph width in full screen on a desktop screen. Could you point me to the correct CSS? These are for the entire site:

–> I would like to make the paragraph width much, much narrower when using the full page width template on a desktop screen (not tablet or phone). That will be much easier for the reader so they don’t have to read all the way across the screen.

Thanks for your help!