I bought this for my 16 year old son. He is 6'2" and slim, in trousers he needs a 34" length. I followed other reviewers who said these were large fitting and got a medium. It's quite a snug fit but he says it's comfortable. The legs are a little short for him but he has long legs.
It's a very thick, fleecy, warm onsie. It looks and feels good quality. I am very pleased with it. In fact I want one for myself.