The ‘ViewportSize’ action changes the current window’s viewport to a given width and height. This is for changing the viewport at some point during the journey and not for setting the overall viewport size. If you want a journey to be entirely in a custom viewport you can change this in the ‘Setup’ panel.
Common reasons for changing the viewport size would be if you are accessing a different site or page with different size specifications to the beginning of the journey. This is also very useful if testing a mobile viewport size compared to desktop.