There's formulas for average size based on wrist measurements as mentioned. Not sure how accurate they are, but wouldnt put much energy into them. As for measuring body parts,all that matters is that you measure them consistently, meaning you use the same technique and positions each time. You can get many different answers on how to measure properly. I measure each muscle flexed and relaxed. I always look to put the tape around the thickest part of a given muscle. The main thing is to keep the tape straight, and make sure it isnt loose. The toughest thing for me to measure is the chest, cant do it myself. The measurements can also vary greatly since its such a large area. I start the tape at the upper arm pit and try to keep it straight while the chest is flexed and lats expanded (for the flexed measurement). For thighs I measure the highest part before the glutes. I chart flexed and relaxed, along with bodyweight and date once a year. Pictures help alot also. All measurements are based on circumference, so the tape goes around the muscles. For a chest measurement that means the back is included. Same with shoulder width. Hope that helps