Tetherball is a traditional grade school game where two opponents stand opposite each other in hopes to securely spin a ball on a rope around a pole. Rules of the official ball size are unclear but can be purchased with a tetherball equipment set.
What we do know about the ball, according to the live strong website, is:
Official tetherballs are approximately volleyball-sized and are available in a variety of colors and textures. Typically firmer than volleyballs, tetherballs may actually sting the hands, so many players choose to use softer models. The tetherball’s most important feature is how it connects to the tether string. Some tetherballs have a protruding loop to secure the tether, while others utilize a recessed bar below the ball’s surface to prevent unnecessary hand injuries.
References:
Livestrong Website – http://www.livestrong.com/article/410559-tetherball-court-size/
Photo – http://saes-de.blogspot.com/2013/09/views-of-week.html
Photo in article – http://www.big5sportinggoods.com/product/outdoor-lawn-games/478242-152360/tetherball.html