You are unable to adjust the SAS® Viya® add-in size (that is, change the width) when using the new Outlook.
For example, this issue occurs when you complete steps similar to the following:
This issue is a limitation of Microsoft. Microsoft did not plan to make this task pane resizable.
To work around this issue, switch Outlook to the "traditional" view. In the traditional view, you can adjust the width of the SAS pane.