Short answer: Measure your chest and shoulders, then compare with the size chart. Choose a fit that allows comfortable movement and light layering.
- Chest: measure around the fullest part of your chest.
- Shoulders: check shoulder seam to shoulder seam if you have a jacket that fits you well.
- Sleeve: measure from shoulder seam to wrist.
- Layering: if you will wear a hoodie or thick sweater, consider sizing up.
Rule of thumb: Leather should feel snug at first but not restrictive.
If you share your jacket material and where you plan to wear it, I can suggest a safer option.