The widget will automatically adjust depending on the size of the container you embed it in.

Normal Width: 540px 

Minimum Width: 400px

If you embed the widget in a container that is smaller than 400px wide, the widget will automatically turn into a button that takes users to the responsive landing page:

To enable this feature, go to the Code/Install tab on your campaign summary and check the 'Take mobile users to a hosted landing page' option: