Just like sc2 button file, its size is 76x76.
However, Photoshop can only generate a DDS file with power of 2 such as 64x64. My problem is, how to generate a DDS with any size?
I normally use Gimp and Image > Scale Image to the res that I need then File > Export As a .dds extension. (click the link in-between the width & height dimensions to unlock them from each other).