Why is Terrain Stamp Brush size limited to 500?

The brush can currently only modify a 2x2 Terrain tile area in one operation; this limits the brush world size to the size of one Terrain tile. There is a change in the works to convert the brush operations to work across any number of Terrain tiles; this change will increase the brush limit to be bound only by the maximum render texture size.

 

That's the default brush size for our brush controllers that come with the TerrainTools package. There's a dropdown to the left of the slider that you can expand to show the min and max values for the brush size slider (and the other brush control sliders). You should be able to remove the 500 world space brush size with that