I guess my dogs are "medium".

Sequoia and Pepper weigh twelve pounds, Vanilla and Karma weigh about thirty pounds, and the other four weigh twenty-something pounds.

Don't know if one would call them "small" or "medium".

Thirty-something pounds is about the upper limit for me. I like to be able to easily pick them up and carry them, if I had to. And ten pounds is probably the lower limit. I would worry about stepping on or breaking an ultra-tiny dog.