Use the Dynamic Outsides toggle to automatically add the Border Size to the current openings, and automatically adjust the outside size as necessary.

Dynamic Outsides automatically adds the Border Size to the bounding rectangle of the current openings, and adjusts the outside size as necessary.  

The Dynamic Outsides Toggle appears bright when toggled on.

The Borders Properties indicate whether the Borders are added to the Top Layer or the Bottom Layer of the current objects when creating the new outside dimension.

When Dynamic Outsides is Toggled On, dragging a new opening to the edge of the mat will adjust the outside size by adding the Border Size around the bounding rectangle of the existing openings.

When Dynamic Outsides is Toggled Off, dragging a new opening to the edge of the mat will not readjust the outside dimensions; the outside dimension will remain a fixed size.

The default setting for the Dynamic Outsides can be set in the Admin > Editor section.