That's a curious thing to be certain of. I can go to 20,000ft or so. How many watts are required to guarantee the statement? It has to be a finite value.
For me that is not 'curious' at all, but since you asked here are the numbers.
Let's assume a distance to be covered of 500 km (well over 250 nm, enough for a visual horizon from 40000' +) at the highest available frequency to us in the VHF comms band, 136 MHz
The free space loss is calculated with the formula
L = 32.5 + 20log F + 20log d = 130 dB
L is Loss in dB
d is distance in km
F is operating frequency in MHz
Let's also assume a less than perfect antenna/cable combination in the aircraft, giving 6dB overall loss, and a slightly better ground installation giving only 3dB loss.
A rather non-sensitive radio could have a sensitivity of 2 uV over 50 Ohms, which equals around -100 dBm.
So in this case your required transmitter power is in the region of:
-100dBm + 130dB + 6dB + 3dB = +39dBm
+39dBm is about 8W
A typical GA VHF radio has 8W carrier power
In the case of a good receiver (0.5uV over 50 Ohm sensitivity , -113dBm) and a better antenna installation on both sides (3dB loss in the A/C, 2dB loss on the ground) this TX power requirement could drop to:
-113dBm + 130dB + 3dB + 2 dB = +22dBm
+22 dBm is about 0.160 W or 160 milli-Watts