This feature was added July 2016.
The default sub grid width is 930px (pre July 2016) or 1000px (post July 2016). To adjust your Sub grids to suit your situation use this Setting to apply the width across the site. You will still need to do your Display forms manually but this setting will save a lot of time with the sub grids.
Note: if you are using Sub-Grids on your Dashboard and do not want the width changed just set each one to a different value than 930 and they will not change.
Setting Name: SubgridDefafultWidth
Setting Value: numeric value
To change one sub individually use the Configuration option in the Appearance section of the Sub grid properties.
This setting is optional. If your sub grid widths suit your situation do nothing.