Specification:

38mm and 40mm common
42mm and 44mm common
40mm, 44mm is suitable for apple watch 5/4 generation
38mm, 42mm is suitable for apple watch 3/2/1 generation
38mm/40mm s/m: 5.5"-8.1" Wrist;
38mm/40mm m/l for 5.9 inch-8.5 inch wrist.
42mm/44mm s/m for 6.1"-8.3" Wrist;
42"/44mm m/l is suitable for 6.5"-8.7" Wrist