The knobs are approximately 6mm in size, the ball itself is filled with air and can be with some force pushing up to about half of what is likely to be responsible for the bouncing ball effect already described, where it dropped from one meter, about 60 to 70cm jumps (when Flummi it was something worse), which is why you first need to worry because more expensive, standing on the shelf ceramic.
As for the quantity and color of the balls: There is a ball delivered (is now but also in the product description) and the somewhat misleading term 'assorted' means apparently only that because in a large stock a large crate is, from the ball is picked up, so the color will surprise.
Unlike the rubber toys this ball does not smell. I mention this only because my little one always quarrels only for a while with the rubber toy until it no longer smells so strong.