How to know what Model and Size of Apple Watch you have?

Your Apple Watch's series and size are printed on the back of the watch.

What is the difference between the  sizes? (38-41 mm and 42-45 mm)

It refers to the size of the Apple Watch's casing or face. Apple Watch models come in different sizes, typically indicated by the diameter of the Apple Watch's casing. The most common sizes are 38mm, 40mm, 42mm, 44mm, and 45 mm, although some older models might have slightly different sizes.

The difference between the  sizes (38-41 mm and 42-45 mm) corresponds to the compatible sizes of the Apple Watch cases. For instance:

38-41 mm: This size range typically refers to Apple Watch models with a smaller casing.These watches are suitable for people with smaller wrists or those who prefer a more compact design.

42-45 mm: This size range likely corresponds to Apple Watch models with a larger casing. These watches provide a bigger screen and are often preferred by individuals with larger wrists or those who prefer a larger display.