Hi-res Landscape Layer Mask

I have a landscape with a heightmap res of 1021x1021 and I want my landscape layer masks to be 2048x2048, however, when I try to import file for layer mask, it will only accept a file the same resolution as the heightmap. Is there a setting somewhere to change this or will I be forced into making a complex material for my landscape to use custom highres textures for my layer masks?