Length of the wire antenna

Hi! I want to do this am radio trasmitter:
http://www.extremecircuits.net/2010/...ansmitter.html
How many inches is it long the antenna wire?

As I know the length of an antenna for a given wave L=lampda/4. if you know the frequency. then wavelength * frequency = speed of light.

I discovered it illegal in Italy. I try with commercial transmitter and receiver.
Thank you!