Well since noone has a Lucas film Boba Fett helmet accuracy is fleeting, but He does have the wavy brow and the uneven mandibles, I have seen noone else do this. As far as size goes, photo's can be enlarged and a reference to something for scale in the same picture that can be positivly I'D. and its size determinded -such as the cheek mole- can be measured to get an exact measurment of the helmet using a plotter.
Correct me if I am wrong but I think BM used something similiar to this technique.