We outline a method by which the angular radii of giant and main-sequence stars located in the Galactic bulge can be measured to a few percent accuracy. The method combines comprehensive ground-based photometry of caustic-crossing bulge microlensing events, with a handful of precise (~10 μas) astrometric measurements of the lensed star during the event, to measure the angular radius of the source, θ*. Dense photometric coverage of one caustic crossing yields the crossing timescale Δt. Less frequent coverage of the entire event yields the Einstein timescale tE and the angle of source trajectory with respect to the caustic. The photometric light-curve solution predicts the motion of the source centroid up to an orientation on the sky and overall scale. A few precise astrometric measurements therefore yield θE, the angular Einstein ring radius. Then the angular radius of the source is obtained by θ* = θE(Δt/tE) sin . We argue that the parameters tE, Δt, , and θE, and therefore θ*, should all be measurable to a few percent accuracy for Galactic bulge giant stars using ground-based photometry from a network of small (1 m class) telescopes, combined with astrometric observations with a precision of ~10 μas to measure θE. We find that a factor of ~50 times fewer photons are required to measure θE to a given precision for binary lens events than for single-lens events. Adopting parameters appropriate to the Space Interferometry Mission (SIM), we find that ~7 minutes of SIM time is required to measure θE to ~5% accuracy for giant sources in the bulge. For main-sequence sources, θE can be measured to ~15% accuracy in ~1.4 hr. Thus, with access to a network of 1 m class telescopes, combined with 10 hr of SIM time, it should be possible to measure θ* to 5% for ~80 giant stars, or to 15% for roughly seven main-sequence stars. We also discuss methods by which the distances and spectral types of the source stars can be measured. A by-product of such a campaign is a significant sample of precise binary lens mass measurements.
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