Version(s): 10, 11
The setting can be setup before drawing the Area by going to File > System/ Drawing options > Material Dimensions > Material Options. Change the Structural Slab Locate option to Exterior/ Bearing. Then trace the area and when a Cleanup is performed it will snap to the Exterior or Bearing within the wall.
Version(s): 12 +
In Area mode, after you have calculated the area you can edit that area to measure to the Exterior Edge, Outside Bearing Edge, or Interior Edge of the walls. Select Edit > Edit Item > click in the area > place a check mark next to the appropriate Reference in the Area Dialog > OK. The area will be automatically recalculated.