According to the University of Minnesota Extension, you should use a bigger sized exercise ball. So, while for exercising purposes you would use a 45 cm ball if you are 5 feet tall, for sitting at a desk you should use a 55 cm exercise ball.How big is a yoga ball?
Use your measurement to get the yoga ball in an appropriate size. If you are 5 feet (1.5 m) or shorter (152.4 cm), you will want a ball that is 18 inches (45 cm). If you are 5 feet (1.5 m) 1 inch (155 cm) to 5 feet (1.5 m) 7 inches (170 cm), try a 22 inch (55 cm) ball.