a signal is to be transmitted through a wave of wavelength lambda using a linear antenna the length l of the antenna and effective power radiated Peff will be given respectively as (k is constant of proportionate)
Range of transmitting antenna can be written as √(2.h.R), where h= height of antenna and R= radius of the Earth.
Length of the antenna is comparable to lambda. So answering the first part of the question, length l = lambda.
Also, effective power radiated is inversely proportional to square of the wavelength. So power P eff = k.(1/lambda²)