Plastic buckles are often used as fasteners for backpack straps, allowing for easy adjustment and secure closure.

And they have many sizes, such as below and also see the attached pictures:

Plastic buckle width 3/4inch, fit the 3/4-inch wide strap Webbing  3/4inch =20mm

               Plastic buckle width 1 inch, fit the 1-inch wide strap Webbing  1 inch =25mm

               Plastic buckle width 1.2 inch, fit the 1.2-inch wide strap Webbing  1.2 inch =32mm

               Plastic buckle width 1.5 inch, fit the 1.5-inch wide strap Webbing  1.5inch =38mm or40mm

               Plastic buckle width 2 inch, fit the 2-inch wide strap Webbing  2inch =51mm