The bodies of physically attractive people follow certain ratios very closely. These ratios determine things like, how far their eyes would be, how long their arms, legs... In fact every proportion for the human body follows certain ratios.
Knowing that; you could measure the physical dimensions of the individual and compare the measurements to the established ratios and the individuals with the least deviation from those ratios would be the most physically attractive. The ones with the greatest deviation would be the least attractive.
The deviation therefore would be your measurement of physical attractiveness.
In regards to other features/traits of the individuals such as hair color, eye color etc - those would be where individual opinions enter the equation. Certain people might have a fondness for blonde hair, blue eyes while others would prefer brown hair, brown eyes but all people would have a fondness for people who deviate the least from the ratios. So for that reason any trait but the physical dimensions is irrelevant in getting a measurement of beauty.