Here's the description from one of the platform tennis sites.
The ball is solid foam rubber, 2 1/2 inches in diameter. It's quite bouncy, and it feels heavy on the paddle, although it weighs only 70-75 grams (around 2 1/2 ounces). Lacking any fuzz, it responds less to spin than a tennis ball does while in the air, but spin affects its interaction with the court surface comparably to what you'd expect in tennis.
It's a big game here in Connecticut.
Viking Platform Tennis balls