When in doubt I always go by the default files. I just opened an FS9 default building texture in DXTBmp.* It's 512 x 512 in DXT1 BMP format. Others are in the 256 x 256 x 8 bit (256 colours) format used in earlier versions of FS. Not sure but I think night textures have to be in this format.

*PS. I assume this is a default texture. The file name & format are shown at the top.