"the sizing on the package says one size fits all and it's 0-6 months. I have a smaller than average 3 month old. I was worried it might be way too big on her since she is only 11 pounds. It's entirely too SMALL. I cannot image how a 6 month old would fit! I can't even pull up the zipper the entire way, and the button on the top of the zipper won't clasp because there is so much tension on the fabric since she's too big for it. Also, the hat at the top doesn't come up far enough to go even halfway up her head. It's like a premature infant costume. My baby has to scrunch up into it and hates not being able to stretch her legs. Also, the material is made of polyester and doesn't breath at all so when I had her in it for 5 min to try to see if I could situate her body better into it, she was sweating."