Forensic anthropology techniques allow researches to make guesstimates of an individual's weight, for example by looking at the amount of wear and tear on joints which occurred before death. It's not going to be exact, but it arguably more objective than looking at other sources, such as portraiture, which could be subject to cultural forces such as exaggerating certain physical features due to the fashions of the time.

There are places on bone where muscles attach and if a person is more muscular, then those attachments are thicker and larger, indicating greater muscat ire. The bones will show signs of malnutrition or good nutrition based on their composition and shape. The length of the femur is a good indication of height. If you've got more bones, then length of femur relative to other bone scan indicate amount of protein when growing up.

One way would be to look at the type of clothing/armour they wore, and the tools/weapons they carried. Armour has to fit properly, and expensive Armour like full plate was tailor made for it's wearer.