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M subdwarfs are low-metallicity M dwarfs that typically inhabit the halopopulation of the Galaxy. Metallicity controls the opacity of stellaratmospheres; in metal poor stars, hydrostatic equilibrium is reached at asmaller radius, leading to smaller radii for a given effective temperature. Wecompile a sample of 88 stars that span spectral classes K7 to M6 and includestars with metallicity classes from solar-metallicity dwarf stars to the lowestmetallicity ultra-subdwarfs to test how metallicity changes the stellar radius.We fit models to Palomar Double Spectrograph (DBSP) optical spectra to deriveeffective temperatures ($T_\mathrm{eff}$) and we measure bolometricluminosities ($L_\mathrm{bol}$) by combining broad wavelength-coveragephotometry with Gaia parallaxes. Radii are then computed by combining the$T_\mathrm{eff}$ and $L_\mathrm{bol}$ using the Stefan-Boltzman law. We findthat for a given temperature, ultra-subdwarfs can be as much as five timessmaller than their solar-metallicity counterparts. We present color-radius andcolor-surface brightness relations that extend down to [Fe/H] of $-$2.0 dex, inorder to aid the radius determination of M subdwarfs, which will be especiallyimportant for the WFIRST exoplanetary microlensing survey.

Radii of 88 M Subdwarfs and Updated Radius Relations for Low-Metallicity M Dwarf Stars