  Stellar properties are measured for a large set of Kepler Mission exoplanet
candidate host stars. Most of these stars are fainter than 14th magnitude, in
contrast to other spectroscopic follow-up studies. This sample includes many
high-priority Earth-sized candidate planets. A set of model spectra are fitted
to R~3000 optical spectra of 268 stars to improve estimates of Teff, log(g),
and [Fe/H] for the dwarfs in the range 4750K<Teff<7200K. These stellar
properties are used to find new stellar radii and, in turn, new radius
estimates for the candidate planets. The result of improved stellar
characteristics is a more accurate representation of this Kepler exoplanet
sample and identification of promising candidates for more detailed study. This
stellar sample, particularly among stars with Teff>5200K, includes a greater
number of relatively evolved stars with larger radii than assumed by the
mission on the basis of multi-color broadband photometry. About 26% of the
modelled stars require radii to be revised upwards by a factor of 1.35 or
greater, and modelling of 87% of the stars suggest some increase in radius. The
sample presented here also exhibits a change in the incidence of planets larger
than 3-4 Earth radii as a function of metallicity. Once [Fe/H] increases to
>=-0.05, large planets suddenly appear in the sample while smaller planets are
found orbiting stars with a wider range of metallicity. The modelled stellar
spectra, as well as an additional 84 stars of mostly lower effective
temperatures, are made available to the community.
