Apple Watch is available in the 6 different sizes 38, 40, 41, 42, 44, and 45mm. The straps for the Apple Watch 42, 44, and 45mm fit on all three watches. So, if you have an Apple Watch 45mm, you can still buy a 44 or 42mm strap. The watch strap attachment still fits on the Apple Watch 45mm. That's also the case the other way around. You can also use Apple Watch 42mm straps for Apple Watch 44 or 45mm. You have a lot of choice for the watch strap of your Apple Watch.