Stellar properties are measured for a large set of Kepler Mission exoplanet candidate host stars .
Most of these stars are fainter than { 14 ^ { th } } 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 \sim 3000 optical spectra of 268 stars to improve estimates of T _ { eff } , log ( g ) , and [ Fe/H ] for the dwarfs in the range { 4750 \leq T _ { eff } \leq 7200 } K. 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 T _ { eff } \gtrsim 5200 K , 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 R _ { \oplus } } as a function of metallicity .
Once [ Fe/H ] increases to \geq { -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 .