This is similar to the existing Unity Terrain Stamp Tool, but which supports special mouse handling and other features.

Special Mouse and Keyboard Shortcuts:

• brush resize: ctrl + mouse drag left/right
• brush rotation: ctrl + mousewheel
• stamp resize: ctrl + shift + mousewheel

Stamp 3d Preview

Inspector:

Example (resize brush, resize stamp, rotate brush):

Example (3d preview)

Unity 2018.3+

Full credit and a BIG THANK YOU(!!!) to the very skilled and most awesome developers at Unity who provided the Terrain Tool Samples for free for the community.