Hello everyone. I had a question about the sizing on the Monkey Daze harnesses...the Angel wing one. I'm not sure which size would fit my baby the best. I have purchased a few items from this brand and there isn't much consistency in the sizes. Last year I got a little snowsuit from a seller on here that is a size small and it fits her perfectly. The other day I ordered a little dress that has a denim skirt, a little Barbie dress and a hoodie. One fits her snug in the chest and the other one is really large and the hoodie was just right.

They are both smalls. So I'm confused as to what would fit. On the chart, it says she should technically be a medium, but that's not correct since she does fit into a small and sometimes it's too big for her. But since a harness won't stretch, I need to be 100% sure.
Her chest measurement is 13.5 inches and she is 9.5 inches long. Which size would be best for her? An x-small, small or like the chart says, medium? Thank you for your time and help!!!