I’m facing the same problem and I’m sorry to inform you that your answer is not clear. =p
If I apply the material BEFORE importing the terrain heightmap, everything fits perfectly and every texture is placed where the masks tell them.
If I apply the material after importing the terrain heightmap, then applying texture maps result in the “filesize does not match with current Landscape extent” error.
How is knowing the current terrain resolution going to help me? Do I have to rebake all the maps to that resolution (madness!)? I tried, it doesnt work, I get the same error.
Thank you.