Here is a short tutorial on how to change the email's width:

To change the email's width, select the body of the email of which you want to change the width.

Use the switch to change the width.

Be careful because 600 px is still the safest email width these days.

Emails that are wider than 600-650 px might not fit into your subscribers' email reading pane in Outlook or Yahoo Mail on smaller screen sizes like 1366x768.

You can also change the block's width:

Please contact us if you have any questions.