The recently measured angular diameter of Barnard's star, together with its large and precise parallax, and a spectral energy distribution that extends from the near-ultraviolet to almost 12 μm establish some of the star's fundamental properties—we find a bolometric luminosity L = (3.46 ± 0.17) × 10-3 L⊙, radius R = 0.200 ± 0.008 R⊙, and effective temperature Teff = 3134 ± 102 K. Accurate knowledge of those parameters helps in turn to constrain the star's metallicity and mass. Although it is evidently possible to estimate bolometric fluxes with good accuracy from photometry alone, angular diameters present more of a challenge, and we examine alternative methods for determining them, namely, through the use of the Barnes-Evans relation and the infrared flux method. We find further evidence that even "state-of-the-art" M dwarf models, which appear to yield good results for the effective temperatures, nevertheless underestimate the radii of the actual stars.