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 R _ { \sun } .
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 .