A jacket that fits well keeps you warmer and moves with you. A poor fit can leave cold gaps, restrict movement or crush the insulation. A few quick checks in the changing room go a long way.
Try the jacket on over the clothes you would normally wear underneath, such as a sweater or fleece. Zip it fully and check that it closes without pulling across the chest or stomach.
Move around. Raise your arms above your head and reach forward: the hem should not ride far up and the sleeves should still cover your wrists. Turn your head to make sure the collar and hood do not choke you or block your view.
Check the length too. A hem that covers the waistband keeps drafts out, and sleeves that reach the base of your thumb leave room for gloves to overlap them.
