Here's a question just for curiosity.
You measure mass on scales. You could measure volume by submerging in a bath with a snorkel system. But how could you measure a living human's surface area without harming.
Ideas so far:
Using software to make a 3D model from images. Effective, but very high-tech.
Wearing a morph suit and rolling in burrs until completely coated, then working out how many were picked up.
Standing naked in a room of known volume for a set amount of time and recording the temperature change in the room. Would rely on knowing that the skin was uniformly at 37 degrees - could you reliably ensure this?
Preferable solutions would be simpler to implement even if a bit approximate.