Answer:
Ogres in World of Warcraft can vary in size greatly - this also seems somewhat related to their level, since lower-level Ogres are smaller.
On average, an Ogre stands two meters (around 7') tall at the young adult stage, but will contrinue to grow throughout their lives. It is therefore quite possible to see ogres that are well over three meters (10').
The largest species of Ogres known are the Gronn, one-eyed Ogres from Outland which descend from Gruul the Dragonkiller. These stand between 3 and 4 meters tall (10'-13').