When the Base Size brush setting is set to 1, the terrain editor doesn’t create any new terrain with the Add tool.
Here’s a video of me using each terrain brush at 1 Base Size.
This happens 100% of the time, expected behaviour is to let me use the tool if it’s set to 1. If there’s no way it can generate terrain at that size, it should indicate it with a message on the screen like the following:
This bug has occurred since the Terrain Editor beta has been able to be enabled in Studio I believe.