The probe is about 1 inch long and less than 1/4" thick. It is cylindical, not ball shaped. The ball shape you may be referring to is the suction cup that can be used to hold the probe in place. I cut the suction cup off and used metallic tape to secure the probe to my heat tape.
I like mine and for less than $30, how can you go wrong for an on/off thermostat?
Jim Smith