Color Map, Detail Map and Light Map Resolution

In the New panel, you may find the drop-down lists, Colormap, Detailmap and Lightmap in the Texture Size group. These fields define the size of the three separate textures for mapping to the terrain.

Colormap is the source of your texture paint brush, the smaller the size is, the less detail it contains when painting on the mesh. If you have a higher resolution image source, you might choose the corresponding size to represent it's full detail. However, if you have the source color texture file in 256x256 pixels size, then Colormap 256x256 is sufficient to represent the full fidelity.
In the texture library, we have a mixture of different sized color textures for EarthSculptor. We may start from 512x512 as the default setting for Colormap for basic terrain texturing.

Mapping Theory for Colormap, Detailmap and Lightmap

1024x1024 texture maps to 513 x 513 terrain (Texture is shrunk to fit in)	256x256 texture maps to 513 x 513 terrain (Texture is expanded to fit in)