In the designer's top bar use the three device icons – desktop, tablet and mobile – to resize the Live Preview to each screen width. The Live Preview toggle on the right switches between the interactive preview and a plain module list. Preview opens the finished email in a new browser tab, which you can also narrow to simulate a phone. Designs are built as responsive email tables; new templates default to a Width of 640 px in Settings, which is a good baseline for most email clients.