The most common jacket zipper sizes are 3, 4, 45, and 5. These numbers are based on the approximate width of the closed zipper teeth in millimeters, so increasing numbers correspond with increasing size.

The size is the first number on the back of the slider (reference image #2). Sometimes the size is also also on the the zipper box (reference image #3).

If there's no identifying number (or the slider is missing) measure the width of the closed zipper teeth in millimeters.

Size 3 ~ 3 mm, Size 4 ~ 4 mm, Size 45 ~ 4.5 mm, Size 5 ~ 5 mm