It is about 23 cm long (from head to tail), 29 cm high, and 35.5 cm wide (from wingtip to wingtip). A Patronus is an advanced form of magic, a powerful defensive spell that only the most skilled wizards can conjure. Harry Potter is, of course, an exception and was able to cast his first spell at the age of 13. They protect the wizard or witch from Dementors by projecting everything that Dementors feed on: hope, happiness, and the desire to survive. Unlike humans, however, they are unable to feel despair, which neutralizes the effects of Dementor attacks.