  Our survey of long-baseline infrared and optical interferometry measurements
is producing considerable numbers of directly determined stellar angular sizes.
We use our sample of 124 high-precision (5\%) angular stellar diameter values
and correlate them with stellar magnitude values from the literature to produce
empirical relations for main-sequence stars between observed apparent
magnitudes, stellar colors, and angular sizes (surface brightness relations).
We find a significant dependence on stellar metallicity for ($B-V$) colors. The
scatter in the calculated relations is small ($\sim$5\%), which makes them a
robust tool for the prediction of main-sequence stellar angular sizes based on
photometry. We apply these relations via the calculation of the radius of the
multiplanet host star GJ 667C.
