Sortd shows a grey tab button on the right hand side of the Gmail screen (mid way down). Clicking the button will open Sortd into Fullscreen Mode. The dropdown arrow provides a menu which allows you to select 'Full Screen', 'Sidebar' or 'Hide'. ‘Sidebar’ puts Sortd into Sidebar mode where as ‘Full screen’ opens Sortd in a maximized view mode.

Alternatively, if you refresh your Gmail tab you will notice an option to start in Sortd mode mode or Gmail mode.

In Sortd Full screen mode the tab will appear red.  In Gmail, if the tab is grey it means that Sortd is not logged in.