Office 365 (Outlook 2013 / 2016 / 2019 for Win) - Minimize or Expand the Ribbon
To gain back some screen space, you can collapse the ribbon by right-clicking on any blank space within the ribbon. Select Collapse the Ribbon. Or, you can click on the ^ symbol at the far-right of the ribbon.
TIP: To quickly go back and forth between expanded and collapsed, double-click on any of the Outlook tabs at the top.