This is a link to the FIBA rules which states (p.12) that the size of the ball must be of a maximum of 78 cm (30.7 inches) like you said and a minimum of 74.9 cm (29 1/2 inches), which is exactly ...

Size 5: 27-28 in. circumference, weight 400-450 g. Futsal balls are also generally a little harder and with less of a high bounce, making them more suitable for hardcourt surfaces and encouraging ...