I have a Series 6 that I think is 40 mm, so the new one would be 2 mm larger, which would be fine. My battery is at 80% max capacity, the threshold where Apple recommends replacement, and I have to recharge often. I'd consider the Series 10, but I'd rather not give up the ability to measure blood oxygenation. I didn't hear if the O2 sensor still exists on the new model. Apple remains involved in a lawsuit with Masimo - at the rate these things go, my O2 sat will probably be 0% by the time it's settled.
Interestingly, Masimo CEO is close friends with Joe Biden, according to CNBC. I should remind the Dems of that the next time they ask for money, which happens about once every hour.