After adding a zipper to a garment, you can change its length.
To resize a zipper:
On the Main toolbar, click Select.
On the Horizontal contextual menu, ensure Edit Points is selected.
In the 2D window, click the zipper. A magenta circular point is displayed at the end of the zipper.
Drag the point to where you want the zipper length to be.
The following image displays zipper resizing in action.