The size of a tennis ball. The mass of the Earth.
But that could change soon.
Current gravitational wave observatories are sensitive to the mergers of stellar-mass black holes. We’ve observed a few mergers involving neutron stars, but most have been between black holes on the order of tens of solar masses.
We can’t yet observe the gravitational waves of supermassive black holes in other galaxies, nor can we observe those of planet-sized worlds. Proposed detectors such as eLISA will allow us to observe the former, but it will take a novel new idea to detect the latter.