If a square meter (SQM) measures 1 mt x 1 mt, you can calculate H with the Pythagorean theorem. The result is some close to 1,41 mts.
Also the height of citizen enter in exactly 1 sqm. Using this we can got:
Height of some mobs:
Citizen: 1,41 mts
Demon: 2,09 mts
Cat: 0,36 mts
Feversleep: 2,09 mts
Nightstalker: 1,59 mts
Sight of Surrender: 1,59 mts
Dragon: 1,77 mts
Ferumbras: 1,59 mts
Giant Spider: 1,32 mts
Jaul: 1,73 mts
Gnomevil: 1,91 mts
Azerus: 1,77 mts
Leave your message with what mob or item you want to know its height.