The Colosseum is not a circle so it has 2 radii. The minor radius is 171/220 furlongs and the major radius is 31/33 furlongs (342/11 rods and 1240/33 rods, respectively).

Update(2/17/06): Thank you, Dan, for pointing out that all the measures listed are actually the diameters (BWL) so make sure to divide by 2 for the radii.