I got the 12R for my 11 year old son who is 78 pounds and 5 feet tall. He is on the slim side and this shirt for him well. The sleeve length was just a little bit long which is a plus because he’s still growing so he’ll be able to wear it for a little bit longer. My son wire this for a band performance and the shirt stayed put while he was moving around playing drums and other percussion instruments. The ease of movement while wearing this shirt is a big plus, and the material is not stuff. Definitely a good buy!