A comfortable fit Apple Watch band should neither be too tight nor not enough. If you have a large or small size wrist, you may need to select an extra-long or extra short strap. If you have an average size wrist, you can pick a standard-length strap.
Measure your wrist by using seamstress tape. Wrap the tape around your wrist and take measures.
If you don’t have any tape or ruler, you can use Apple’s print out guide to measure your wrist and it doesn’t require any extra tool other than access to a printer.
Don’t measure the strap lengths with the buckle. Measure the length of both straps together in millimeters (mm). You can measure two-piece straps one by one and add them up to get the total strap length. If your current Apple Watch band is a suitable length, you should purchase the same length of the strap.
Note: If you want to purchase an Apple Watch case, just make sure your Apple Watch size and the size of the case.