Most people will find a fit by ordering the size they usually wear in men’s clothing. If you are sometimes between sizes, I’d advise choosing the smaller.
FirstGear publishes a chart which summarizes the max chest and max waist/belly for each size and we’ve included that below. I found this jacket to match up to the chart pretty well. I think the max belly figures below are a touch smaller than the actual jackets measure, so I’d say the max belly for any given size is about 4” less than the max chest. For example, in a size Large, the max chest is 44” and the max belly is about 40”.
| Size | Small | Medium | Large | XL | 2XL | 3XL |
|---|---|---|---|---|---|---|
| Max Chest | 34-36 | 38-40 | 42-44 | 46-48 | 50-52 | 54-56 |
| Max Waist/Belly | 28-30 | 32-34 | 36-38 | 40-42 | 44-46 | 48-50 |