In the Inspector window, switch to the third tab, then open the Brush foldout, you will see the properties as listed below:
Mode: which mode to use, Spawning or Masking?
Radius: radius of the brush.
Density: determine how dense to spawn objects.
Scale Min: minimum scale of spawned objects.
Scale Max: maximum scale of spawned objects.
Max Rotation: maximum rotation of spawned objects.
Combination Physical Size: size of an object combination. Larger value allows more objects to be grouped into one combination. Choose it wisely, too small value requires more draw calls to render the scene, while too large value loses the advantage of culling.
Keep Colliders For Groups: if turned on, grouped objects will have colliders like its prefab, otherwise its colliders will be removed.