I'd say an ogre (GW-sized, anyway) would be Huge – even GW humans stand pretty large!
My rule of thumb with 28mm figures is: Big = 2–3m (as per the rules), so essentially 30–40mm.
Huge is therefore 40mm+. Where you cut off is essentially up to you and your opponents, but I'd probably say 50mm is the upper end for Huge (Gargantuan thus coming in at 50mm+). Remember that size comes with disadvantages as well as advantages!