Ideally the ground plane for a monopole whip antenna is a circle with the monopole ship in the center. The circle should have a diameter equal to lambda/4 (1/4 of wavelength).
However in practical designs this is rarely possible. So the general rule of tomb apply: Try to keep ground plane as large as possible.
Last Update: August 8, 2017
January 11, 2017 1796 Antenna Q&A