I ordered a size up from his old Hanes tanks and was surprised they were a little larger than expected. Never going back to hanes after these.  Theyre great quality and run true to size. Theyre much softer and the cotton is wonderful. My son is 8yr old and weighs about 69 lbs, hes tall w/ a med build. He usually wears a size medium in hanes tanks and an 8/10 in tops. Hope this helps.