  The precisions of extrasolar planet radius measurements are reaching the
point at which meaningful and discriminatory comparisons with theoretical
predictions are can be made. However, care must be taken to account for
selection effects in the transit surveys that detect the transiting planets for
which radius measurements are possible. Here I identify one such selection
effect, such that the number of planets with radius R_p detected in a
signal-to-noise ratio limited transit survey is proportional to R_p^alpha, with
alpha~4-6. In the presence of a dispersion sigma in the intrinsic distribution
of planet radii, this selection effect translates to bias b in the radii of
observed planets. Detected planets are, on average, larger by a fractional
amount b ~ alpha (sigma/<R_p>)^2 relative to the mean radius <R_p> of the
underlying distribution. I argue that the intrinsic dispersion in planetary
radii is likely to be in the range sigma = (0.05-0.13)R_J, where the lower
bound is that expected theoretically solely from the variance in stellar
insolation, and the upper bound is the 95% c.l. upper limit from the scatter in
observed radii. Assuming an arbitrary but plausible value of sigma/<R_p>~10%,
and thus b~6%, I infer a mean intrinsic radius of close-in massive extrasolar
planets of <R_p>=(1.03+/-0.01)R_J. This value reinforces the case for HD209458b
having an anomalously large radius, and may be inconsistent with coreless
models of irradiated giant planets.
