I measured my “bust” then added a bit so I would be comfortable in the vest with a sweatshirt under it. I used the size chart on the website using the measurement for the bust as the deciding size.
The vest has two adjustment inside that allow you to reduce the size. They are nice tape adjustments, but work like the adjustments on a hoodie. You can pull on either side then the tape “velcros” flat. The adjustments are just below or at the bust line and at the the waist. They adjust the size down a pretty fair amount but I can’t figure out a way to measure the change. I think the honeycomb padding makes this vest adjust well.
I’m sure I saw a measurement chart on the website that gave bust, waist and over the shoulder dimensions,