Thumbs up! An albino pygmy marmoset monkey born in late August got a big welcome at Froso Zoo, in Ostersund, Sweden. At full size, the world's smallest monkeys can weigh up to 3.5 ounces and measure 13.7 inches long. Found in the wild in South America's upper Amazon basin, the petite primates communicate using high-pitched clicks, whistles and squeaks.