Halo Sleepsacks sizing?

So my little girl is 6 months but big for her size 28 inches. So we have sleep sacks we were given when she was born that are supposed to go up to 9 months but she is already too long for them. I was looking at the Halo sleep sacks and they are small medium and large and there is no sizing chart on the site I am at. I did find one but I wanted to know how true to form is it. So anyone that has one please tell me your opinions on the sizing and if they are worth it. I would get the fleece as we live in VT and it is cold.