A standard watch band size is 20 cm, although there are models of 18 or 22 cm. What changes is the end of the bracelet, which can be a short strand (8 cm) or a long strand (12 cm). The width of the bracelet (as well as the width of the buckle) is more important when choosing since it is the one that will allow to fix it to the lugs.
Changing a watchband can be a bit technical, requiring the use of tools, especially for removal. Some major brands whose watches are still under warranty even require that the strap be changed in a boutique or at an authorized dealer. With a Nato bracelet, it's a different story: replacing it takes about 15 seconds!