  We have measured the angular diameters of six M dwarfs with the CHARA Array,
a long-baseline optical interferometer located at Mount Wilson Observatory.
Spectral types range from M1.0 V to M3.0 V and linear radii from 0.38 to 0.69
Rsun. These results are consistent with the seven other M-dwarf radii
measurements from optical interferometry and with those for sixteen stars in
eclipsing binary systems. We compare all directly measured M dwarf radii to
model predictions and find that current models underestimate the true stellar
radii by up to 15-20%. The differences are small among the metal-poor stars but
become significantly larger with increasing metallicity. This suggests that
theoretical models for low mass stars may be missing some opacity source that
alters the computed stellar radii.
