When you want to increase the amount of space in the Outlook window for viewing messages, calendars, or tasks, you can turn off the folder pane.
Click View > Folder Pane, and then click Off.
To turn it back on, repeat the steps, but click Normal.
You can also free up space in the Outlook window by minimizing the Folder Pane. The slim profile of the minimized Folder Pane gives you more reading area for reading your Outlook items, while still providing quick access to the folders you use most often.
At the top of the Folder Pane, click . To change the Folder Pane back to expanded view, click .
When the Folder Pane is minimized, click a folder name to view that folder’s contents, or click All Folders to see a pop-out window where you can pick any of your folders.