  We present Spitzer 8 micron transit observations of the extrasolar planet
system HD 149026. At this wavelength, transit light curves are weakly affected
by stellar limb-darkening, allowing for a simpler and more accurate
determination of planetary parameters. We measure a planet-star radius ratio of
R_p/R_s = 0.05158 +/- 0.00077, and in combination with ground-based data and
independent constraints on the stellar mass and radius, we derive an orbital
inclination of i = 85.4 +0.9/-0.8 deg. and a planet radius of 0.755 +/- 0.040
Jupiter radii. These measurements further support models in which the planet is
greatly enriched in heavy elements.
