My wife is using it with a 4" screen, since that's the only device in the house she can use it with.
We have an (old, useless) Apple TV, a few iPhones, and 2 Macbooks. I'm not going to buy another Apple TV just for fitness+, that's such a waste of the environment. Hopefully Apple comes to their senses soon.
My wife owned an iPad, got rid of it because she didn't like it. Now Apple is punishing her.