  Using the longest baselines of the CHARA Array, we have measured the angular
diameter of the G5 V subdwarf $\mu$ Cas A, the first such determination for a
halo population star. We compare this result to new diameters for the higher
metallicity K0 V stars, $\sigma$ Dra and HR 511, and find that the metal-poor
star, $\mu$ Cas A, has an effective temperature ($T_{\rm eff}=5297\pm32$ K),
radius ($R=0.791\pm0.008 R_{\rm \odot}$), and absolute luminosity
($L=0.442\pm0.014 L_{\rm \odot}$) comparable to the other two stars with later
spectral types. We show that stellar models show a discrepancy in the predicted
temperature and radius for $\mu$ Cas A, and we discuss these results and how
they provide a key to understanding the fundamental relationships for stars
with low metallicity.
