The relationship between length and power of monopole antena

Hi every body, please tell me the the relationship between length and power of monopole antenna. I have a military communications Tx/Rx and It use monopole (whip) antenna. I don't understand why: I have to use antenna 2.4 meters at low power mode (7-14 Watt) and use antenna 3 meters at power amplifier (50 Watt) .

There is no relationship between the power and the physical length of an aerial, other then if you put too much power into a monopole, the current will actually melt it. This is not likely with only 50W. A different length aerial without any further tuning will effect the power output.So it could be the other way around, i.e. a one metre aerial in your case reflects most of the power back to the transmitter so it might only radiate 1 watt.
Frank