Put a ruler, a tape measure, a standard sheet of paper, or anything else of known length into the scene. Then snap a picture of it and use imdistline() to find the length in pixels. For example, let's say you laid down a sheet of A4 paper and measured it's distance as 2100 pixels. Knowing that it is 21 cm long, you get a spatial calibration factor of 21/2100, which you multiply your distances in pixels by to get the distance in cm. Multiply by that calibration factor squared to convert the area in pixels to the area in square centimeters.
calibrationFactor = 21/2100;
distanceInCm = distanceInPixels * calibrationFactor;
areaInSquareCm = areaInPixels * calibrationFactor ^ 2;
See my demo attached below for a really nice demo.