How much should you weigh and how big should your thighs be if you are a 12-year-old?

If you're twelve you may be a little heavier/lighter because you are still growing. It depends on your height, weight, and frame.

Your thigh size can be any size. It depends on your body type, for example, or how much muscle you have. If you have a pear shaped body you'll have bigger thighs (usually) than someone who is an Apple shape and of the same weight/height/frame.