Dashboards now have the width options "Full screen" and "Fixed (960 px)". Up until this update, full screen was the only option.
One important thing to note here is that a report will inherit the width setting the dashboard had when the report was created. This means that you are now able to
a) create reports that have full width b) see how a fixed width report will turn out while designing the dashboard
You can find the dashboard’s width setting in the top pane while in edit mode: