The Medium ball is the standard ball that ships with every Skill Board Mini. It's specially reinforced to help it withstand the weight of the board and rider. 

Similar to a size 4 soccer ball, it measures about 8.25 inches (21 cm) in diameter. The pressure in the ball determines how difficult the ride is. Please see our Getting Started video for more information.

If you're buying a ball without a board, then you will probably also need a pump.

The medium ball is rated for 300 lbs (136 kg)