Hi,
To be in the far field region, you have to make sure that Radiation boundary will be at a distance of atleast Lamda/4 or greater. For some not so strongly radiating structures it far-field region could be achieved at a distance of Lamda/10. But in most cases, Radiation boundary should always be atleast at a distance of Lamda/4 or greater in order to get correct results for Radiation characteristics.
In case of loop antennas,they are classified in two categories,eletrically small and electrically large antennas. Electrically small loop antennas r those whose overall length(circumference)is usually less than about one-tenth of a wavelength(C< Lamda/10). whereas, Electrically large loop antennas have circumference which is about a free-space wavelength(C~ Lamda). Electrically small loop antennas are poor radiators and are mostly used for receiving in applications where antenna efficiency is not important but signal-to-noise ratio of the system is. Whereas electrically large loop antennas are good radiators and mostly used in directional arrays(one prime example is helical antenna).
Now you have to see your antenna dimensions as mentioned above and decide about the Radiation Box size. If you have an electrically small loop antenna then Lamda/10 distance is enough,otherwise stick with lamda/4 or greater distance for radiation box.
Hope it helps.
/SC