Parasitics in the loop - inter turn capacitance in particular - are not taken into account. They will limit the high frequency response of your loop, so the estimate of maximum frequency is probably off.
About the wire radius - this calculator seems to give more realistic results when the "wire radius" is actually half the spacing to the next wire - in other words they seem to have assumed tightly packed magnet wire. The default values given are for my "ribbon cable" loop, which has 26 AWG conductors spaced 0.050 apart. Obviously I did not enter the radius of 26 AWG wire, I entered half the winding spacing instead! I do not know how well this calculator holds up for more widely spaced conductors, but I do know that if you space the conductors more closely, the Q of a loop seems to rise, making it more useful on the high end of the band.