The resolution size of your texture maps must all be the same sized and all must be a power of 2.
Here are some common resolution texture sizes:
128x128 (recommended for LOD texture, if making or baking a custom LOD texture)
256x256 (recommended for LOD texture, if making or baking a custom LOD texture)
512x512
1024x1024
2048x2048 (only use on the largest, most special, and most unique buildings)