When the hardest part to give my petty answer is finding out how fast an average person can throw a tennis ball, people serve them mostly. If you throw it is probably somewhere in 80-120 km/h range. Fastballs of pro's in baseball are 150-170 km/h, serves in tennis are close to the same number only in miles per hour.
There is an infinite number of orbits = orbit velocities your tennis ball can go at, from circle to elliptical ones, till you hit escape velocity and then you have a chance at murdering some innocent alien few million years in the future.
G - gravitational constant ; M - mass of both objects or mass of the planet in our case; R - distance between centers of gravity; g - gravitational acceleration aka. surface gravity.
Table with some fast calculations
So 30-50 km radius for my speeds of 80-120 km per hour. Density used is what you would expect of a nonmetallic asteroid, but a rocky one at least.
Of course, you could make a tennis ball orbit any asteroid with smaller values, just don't throw it too fast or it will escape. You can even let the tennis ball orbit you if you are okay to wait for something around the magnitude of 1-3 months for a revolution or way more if the radius is bigger.
You can increase it by using a hollowed/altered asteroid, you can play with new mass and radius. How big you could build it before it collapses? That is a big question of itself. Just find what asteroid fulfills requirements of your story and then tweak it for this stunt.