According to Guinness World Records, Gibson, a Great Dane, is the world's tallest dog, from floor to shoulder 42.2'. He stands 7'2' on his hind legs. Gibson plays with his friend, Zoie, a 7.5' Chihuahua in an undated photo. A single gene makes some poodles purse-sized while allowing a Great Dane to look a pony in the eye, U.S. scientists reported on Thursday in a finding that may shed light on human size differences and diseases. (Deanne Fitzmaurice/Handout/Reuters) A single gene makes some poodles purse-sized while allowing a Great Dane to look a pony in the eye, U.S. scientists...