We present a synthesis of physical effects influencing the observed lightcurve of an extrasolar giant planet ( EGP ) transiting its host star . The synthesis includes a treatment of Rayleigh scattering , cloud scattering , refraction , and molecular absorption of starlight in the EGP atmosphere . Of these effects , molecular absorption dominates in determining the transit-derived radius R , for planetary orbital radii less than a few AU . Using a generic model for the atmosphere of EGP HD209458b , we perform a fit to the best available transit lightcurve data , and infer that this planet has a radius at a pressure of 1 bar , R _ { 1 } , equal to 94430 km , with an uncertainty of \sim 500 km arising from plausible uncertainties in the atmospheric temperature profile . We predict that R will be a function of wavelength of observation , with a robust prediction of variations of at least \pm 1 % at infrared wavelengths where H _ { 2 } O opacity in the high EGP atmosphere dominates .