between the size of a grapefruit and a basketball, consisting of a 6" diameter plastic sphere with 12 1.5" tapered pentagon-shaped nubbies
spread evenly over the surface (that would put them as the vertices of an icosahedron
or the faces of a dodecahedron
Inside the sphere is a small electric motor, with it's spindle coinciding with the sphere's axis. The motor-battery combination, has its center of mass off-axis so that when the motor runs, the bumbleball's center of mass moves greatly.
in short, it's a brightly-colored plastic ball that hops around. it reminds me (in a word-association sense only) of centripedal bumplepuppy.
See: bumbleball gallery for visuals, and bumbleball of doom for "this, but gone horribly wrong"