Once you have selected your criteria to sort your messages by, you can collapse all of the groups down so it is easier to see the groupings of email.  To do this, first sort by your criteria.  In the example below we sorted by Size.  Then click on the View tab, and select the drop down next to Expand/Collapse and make your selection.  Below we have collapsed the groups.

 

 

 

To go back to your view from before, right click on the heading bar and select Arrange By --> Date to see your email as you had before.