It seems like people are saying 3-6 months is too small, I haven't tested that size out yet, but nb and 0-3 months are both true to size. These really are the best onsies, they're soft and very stretchy! My baby's 2 months, she's in the 5th percentile for both height and weight and the nb's still fit but they're a little snug around her pot belly. 0-3 is a little wide and long on her. Hope that helps.
I buy a set of these for my daughter every year. I buy the short sleeves for summer. They are so soft and light weight. My daughter is so comfortable in them! When I put her in one after her bath, she actually snuggles herself into it. They run large, so I buy a size down from what she normally wears. They wash well, and hold up surprisingly well with continued wear. For the price, they're a great buy.