What size watch band does my watch take?

In the Watch app on your iPhone go to My Watch, then at the top of the page tap All Watches. You will see the entry for your watch, including the case size. This tells you which band size you need to order, for 38/40mm or 40/44 mm case.

38mm and 40mm watch faces use the same band - this is the smaller Apple Watch

42mm and 44mm use the same band - this is the larger Apple Watch