This past February, at Toy Fair, Basic Fun Inc. showed off a new series of Madballs, featuring a mix of old and new characters. Those still haven't been released yet, but there is something new you can get in the meantime.
Italian company Giraprendi has a series of small-scale Madballs that can squirt water. How small? Well, they're sold gachapon-style in capsule machines, so they can't be full-sized; the capsules themselves are listed as 55 millimeters, so that'd be about 2" - that should give you an idea of how big the ball inside it could be.
Here are all the characters:
Click each of them to see them bigger, and learn their names.