A leather jacket should close comfortably over its intended layer and allow the movements you need without strained seams. The desired silhouette can be close or relaxed, but comfort should not depend on dramatic stretching after purchase.
Fit the upper body first
Wear the shirt or light sweater you expect to use. Check where the shoulder shape sits, whether the collar rests naturally and whether the chest closes without pulling the zipper out of line. Shoulder construction varies, so judge the silhouette and movement together.
Reach forward, bend your elbows and sit down. Note pressure at the armholes and upper back. A jacket that looks tidy with arms at your sides can become restrictive when you use it normally. Make the movement test part of the fitting.
Check length and layering room
The hem should suit the style and remain comfortable while seated. Sleeves should provide the intended coverage without obstructing your hands. Cuffs, ribbing and zipper sleeves change how length feels. Try both open and closed positions.
If you need a thicker winter layer, test it now. Sizing up may increase body length and shoulder width as well as chest room. A different pattern can solve that problem more effectively than simply choosing a larger label size.
What signs suggest the size is wrong?
Persistent closure tension, distorted pockets, restricted reach and uncomfortable armholes are useful signals. Extra fabric alone is not proof of poor fit in a relaxed design. Take photographs and compare the garment's measurements with the supplier's chart before deciding.
Related reading
Agreeing jacket measurement methods and tolerances. For another useful reference, see Fitting outdoor team jackets across sizes.
