We make our jackets to fit comfortably over 1-3 layers based on the sizes below. Our girls fit jackets are tapered slightly at the waist.
The most important measurement for the jacket is height - if your child is larger than the height measurement then the sleeves/body may be a bit short, and they will likely grow out of it within a few months. We generally recommend to go with the size up, especially if you want to get 2 years out of the jacket.
Scroll down for diagrams on how to measure your child.