On the back of your Apple Watch, you should see either a 41mm case specification or a 45mm case -- this is if you have the Series 8. Also: Apple Watch 8 reviewThere are four other specifications that align with other models: 38mm, 40mm, 42mm, and 44mm. Knowing the size of your dial is important when purchasing a band because it'll help you figure out how you want your band to look in relation to the dial.