With the new 2.0 release of the PLNAR app, you can measure rectangular structures on a wall such as windows, openings, art, or other items.

Step 1) Make sure you have the perimeter of the room already marked out (see here for instructions) using PLNAR

Step 2)  Mark sure you have defined the height of the floor to ceiling (see here for instructions) using PLNAR

Step 3) Long press (hold down) on the big white button and the measure tool will change to a rectangle.  

Step 4) Now you can just point at a corner, hit the button to start the rectangle, and drag to the opposite corner.