Measuring someone's size in a photo can be a useful tool for scientific research or to estimate how tall someone was years ago. Because a photo is entirely self-contained and can magnify objects, the only true way to measure something is to locate a point of reference. A point of reference in this context is simply something you already know the size of. By measuring the difference in scale of that object, you can successfully compute someone's size.
Locate your point of reference. If possible, use a point that is the same distance from the camera as the subject you're trying to measure.
Measure the size of the object in the physical photograph with a ruler or tape measure. The units you use are up to your preference.
Divide the known height of the object of reference by the physical size of it on the photo. Write this number down; this the scale multiplier.
Measure the physical size of the subject with the unknown size.
Multiply the physical size of the subject by the scale multiplier to produce the actual height.
Some miscalculations can occur if you use a reference that is too far away from your subject.