Nice ork.. I myself do alot of Warhammer :)
The scale, as Games Workshop themselves say, is 28mm, or 1/56th.
Then again, these are meant for tabletop gaming, which means alot of moving of them and such. Therefore they are made thicker, and may look slightly bigger. Just as a Warhammer soldier is bigger than a Tamiya 1/48 soldier, yet the scale says the Tamiya one is bigger.