User Configuration > Administrative Templates > Microsoft Word 2016 > Word Options > Advanced
User
HKCU\software\policies\microsoft\office\16.0\word\options
This policy setting allows you to set the width of the style area that shows the names of applied styles to the side of document text. If you enable this policy setting, you may specify the width of the style area. If you disable or do not configure this policy setting, the default width is used.