Featuring a convenient buckle for easy removal, it will not tarnish, rust or break. Extra links and vinyl comfort tips are available for this training collar. Chain training collars are safe, effective training tools with proper use. They are not to be used for tie out. Small collar: 2.0mm x 14″. Medium collar: 3.0mm x 18″. Large collar: 3.3mm x 20″. Extra Large collar: 4.0mm x 22″.