How to find what size your Apple Watch-Iwatch Band at any time.  It's a simple thing since it is listed on the ceramic back of the Apple Watch Case.  The Series and the watch size in MM are on the top side.  See the photo as and example.  The first number indicates it a Series 4 and the size is a 44mm.  A 38mm iwatch band fits the 40mm and a 42mm watch band fits the 44mm.  Apple did not change the band sizes.  The iwatch watch casing size did change.  It's key that you purchase the correct size watch cover for your apple watch case.  Iwatch Candy carries and lists all sizes.