"LOD diffuse texture '..\Source\TGATextures\lod\ShipLOD.tga' and normal texture '..\Source\TGATextures\lod\ShipLOD_n.tga' are not the same size (but should be)."

THEY ARE!!!

Done this again and again, using default LODs. If I uncheck "Has distant LOD" it is fine, then CK finds another texture and gives same error "...are not the same size (but should be)."

Why is this so difficult?

I got this error even if the textures doesn't exists. Make a double check of your folder paths and texture names. The Source folder must be in the same way as Skyrim folder (not in data).