Using the longest baselines of the CHARA Array, we have measured the angular diameter of the G5V subdwarf {mu} Cas A, the first such determination for a halo population star. We compare this result to new diameters for the higher metallicity K0V stars, {sigma} Dra and HR 511, and find that the metal-poor star, {mu} Cas A, has an effective temperature (T_eff_=5297+/-32K), radius (R=0.791+/-0.008R_{sun}), and absolute luminosity (L=0.442+/-0.014L{sun}_) comparable to those of 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.