Changing width of text area in posts and pages

Hello Dave,

Apologies for asking this very simple question, but I haven’t been able to figure it our myself: ¿How can I change the width of the text in posts and pages? I have my theme set in full width with no side bars and that makes the text occupy the whole width of the content wrapper. I would like to keep the original width of the wrapper but just reduce the are were the text extends in the post and pages.

Thanks a lot