If I were open to sitting, walking, and dropping in for a variety of dog sizes, but would only want to board small dogs for my small apartment, is there a way to indicate that in my Services settings?

I would recommend listing that detail of your service in your About You section, possibly near the beginning so it's one of the first things a potential client sees? Another option would be to change your tagline from "Pups are Life" to something that mentions you can take little boarders. Pups for Life is a cute tagline though, just saying that's another spot you could list it.