I'm often confronted with problems as a physicist where one can only get a
handle on upper and lower bounds.
Lower bound:
I'd say the minimum number and length of radials is 3 (must define a plane)
and 1/4 wavelength (satisfies boundary conditions).

Upper (infinite sheet of copper)
As Walt and Reg have debated, the "Cleese extreme" (to steal from Reg's
post) is trying to duplicate the "infinite perfectly conducting plane" of
our elementary physics books.
Cheers and beers
H.

73, NQ5H