You can use Photometer to measure dimensions of everything within a photo.
Photometer uses a predefined index within the photo and measures the pixel height in millimeters based on index's actual height, after measuring the height of pixels in millimeter you can measure any object's dimensions, by specifying its upper edge and lower edge in the photo.
Objects that being measured by Photometer MUST be at the same depth as index's depth, in other words distance of the object from camera must be equal to the distance of index from camera.

Notice: Devices with android version lower than 4.0 are not supported.