Problem Layer Painting

hi i have a problem with the layer painting because i wanna paint a rock texture and dirt in the ground but if i try he fill a triangular section, i wanna try to do in a very small area not with the brush radius at 1 i wanna do at 0.01. how can i do ?

Not supported by the stock terrain system. Also one square centimeter precision masks would not practical for real-time purposes at all.