Or if you don't want to run an .exe...
http://www.wowpedia.org/EdgeFiles
They are actually split into 8 sections. As such, your dimensions for the file should be 1:8 (while the length and width stays a power of two). So... 8x64, 16x128, etc. Then use an image editor which has a grid you can show and set this grid to divide your canvas into 8 equal sections (with subsections, if you like).