Procyon B is one of a select group of white dwarf stars which are members of visual binary systems and represents a critically important object for testing the fundamental physics of stellar degeneracy. We have accurate measurements of the binary period and the white dwarf mass. However, the extreme brightness ratio and the small separation between primary and secondary makes accurate ground-based determinations of basic characteristics, such as effective temperature and chemical composition, which we require to derive stellar radius, impossible to obtain. Furthermore, since the age of this system is known, a temperature determination for Procyon B constrains its cooling age, providing an estimate of the progenitor mass, a stringent test of theories of stellar mass loss. We present the results of photometry from a series of WFPC2 images, from which we determine an apparent magnitude of mV = 10.92 +/- 0.05, and an effective temperature of 8688 +/- 200 K, based on Hubble Space Telescope equivalent UBVRI filter measurements. Our more uncertain, problematic, narrow filter results argue a helium composition is more plausible for Procyon B. The stellar radius we derive from these parameters, assuming a mass of 0.622 +/- 0.023 M☉, is 0.0096 +/- 0.0005 R☉, inconsistent with the carbon-core, zero-temperature mass-radius relation of Hamada & Salpeter. This implies that Procyon B has a heavier core than carbon, calling into question the assumption of carbon-core composition commonly used for white dwarf stars.