-open WE, and use the smallest Grid (press G a few times =)), you should see grey boxes on the ground now.
-now place your building.
-just count the grey boxes it needs =) (for example its 1 box high and 1 box wide)
-open GIMP (this is a freeware grafic programm, use google ^^) or Photoshop
-Create new file with Size 1 Pixel x 1 Pixel, well u have to insert here the numbers of grey boxes you counted in the WE for the size of your building.
Tpp: use the magnifying glass tool, because 1x1 pixel is just very small =)
- paint your pixels in colour MAGENTA, this is the colour wc3 uses for the normal building pathability
-save and import it to wc3 (tga or blp)
- chose your new pathing map in the WE, u made it =)
Greets