any sitters watch small dogs only?

I haven't been with Rover long enough to build up a sufficient client base, but when I have, I think i might decide to sit for repeat clients only. I don't see anything wrong with doing it that way. The reasons you give are valid.

I also haven't been with Rover long enough to have a full schedule.
It is really all up to you on how you want to run your own business. I also watch only 2 dogs at a time, and they have to be from the same household! Kudos to you for getting to the point of taking care of regular clients and not taking on new clients. Hey, Doctor offices do it, so why not? Seriously, you are the one that has to manage your time and resources. You also know what your limits are as far as stress level, and what would and would not be a good fit for you. As your title question indicates, there are some sitters that limit themselves to small dogs only. I say there is enough business to go around. Especially when we operate in our own niches. I know there is a sitter on Rover that lives near me, but she specializes in Dachshunds, and another one specializes in small to medium dogs. They found a niche that works for them! I market myself as accepting large and extra large dogs for no additional charge. (Same rate applies to all sizes).

Right now I limit myself to only have 2 dogs from the same household staying here at a time. Yes, I could make more money by watching in my home dogs from multiple households. My fear is that they may fight, and my stress level can't handle that. There are other sitters that watch dogs from multiple households and they work very hard to keep an eye on them & have procedures to reduce conflict. I think that is great, but I don't have the facilities to do what I would want to have dogs from multiple households staying with me. Don't feel like you have to apologize for your policies. You have them for a reason, and it is literally your business.