In your screenshot above there’s two black material previews: LandscapeGrass_Inst is the material instance to apply and LandscapeGrass is the parent material to open and edit.
You’ll always need to do this for landscape materials no matter what you’re doing as there’s no way to directly parameterize landscape coord value. I’ll see if I can make it simpler as it’s easy enough to just multiple the coordinates results.
Here’s where you edit the mapping scale:
The default values may also be fairly OK so this step isn’t all that mandatory unless your material looks really bad.