i worked it out to be 470mm or 230mm.
if anyone is wondering how you work it out:
>wavelength = velocity / frequency
velocity is the speed of light
put some numbers in
>wavelentgh = 3e8 / 40 e 6
then you need to keep dividing by 2 until you get a nice manageable length. in this case 7.5m /16 or 7.5m / 32
and it is the same length as the wire on a fuataba!
hope that helps