Problem: Can't set the brush size as small as desired.

Background: The terrain is the limiting factor. The minimum brush size represents what the terrain can still handle. This depends on the size of the terrain versus its resolutions:

  • Heightmap Resolution
  • Control Texture Resolution
  • Detail Resolution

So effectively the terrain's resolution in world space.

This information is also available in the tooltips:

Solution: You can increase the resolutions of the terrain, or make the terrain smaller (so the same resolution applies to a smaller space). This will increase the terrain's world space resolution and allow for smaller brush sizes.You can find these settings under the Terrain Properties tab of the terrain (Detail resolution matters for grass clearing; If grass clearing is inaccurate, you may need to increase the Detail resolution of the terrain):