Apple watch size

Depends which strap/band you wish to order, some are adjustable and some are fixed custom size. Take a look at the different styles at Buy Apple Watch Series 6 Bands - Apple

Solo Loops and Solo Braided Loops are not adjustable and require measurement of your wrist to order the correct size. Sport Loops and Sport Bands are adjustable and do not require a particular size, other than choosing the right size for your watch (38/40mm or 42/44mm).