Erro with brush(Texture Paint)

I'm trying to paint the pupil but the brush is on the smallest size, it's still too big, not even the mask works and I don't know what to do, I'm asking for help please and thank you very much. There are smaller brushes and how to do them too, I downloaded some but when I import them they don't recognize them in this project, if I open a new one it works and I don't understand, if someone helps me I would really appreciate it.