With more than 3,000 different species of walking stick insects, their size can vary widely, according to National Geographic. The smallest is North America's Timema cristinea, which averages about one-half inch in length. The largest, by far, is Borneo's Phobaeticus kirbyi, with a 13-inch body and overall length of up to 21 inches, when the stick stretches out its legs. Female walking stick insects are generally larger than the males.