We calculate directly determined values for effective temperature (Teff) and radius (R) for 191 giant stars based upon high-resolution angular size measurements from optical interferometry at the Palomar Testbed Interferometer. Narrow- to wideband photometry data for the giants are used to establish bolometric fluxes and luminosities through spectral energy distribution fitting, which allows for homogeneously establishing an assessment of spectral type and dereddened V0-K0 color; these two parameters are used as calibration indices for establishing trends in Teff and R. Spectral types range from G0III to M7.75III, V_0_-K_0_ from 1.9 to 8.5. For the V_0_-K_0_={1.9,6.5} range, median Teff uncertainties in the fit of effective temperature versus color are found to be less than 50K; over this range, Teff drops from 5050 to 3225K. Linear sizes are found to be largely constant at 11R_{sun}_ from G0III to K0III, increasing linearly with subtype to 50R_{sun}_ at K5III, and then further increasing linearly to 150R_{sun}_ by M8III. Three examples of the utility of this data set are presented: first, a fully empirical Hertzsprung-Russell diagram is constructed and examined against stellar evolution models; second, values for stellar mass are inferred based on measures of R and literature values for logg; finally, an improved calibration of an angular size prediction tool, based upon V and K values for a star, is presented.