We have constructed a micromagnetic model to study the exchange coupling between Co (a ferromagnet) and γ- Fe50 Mn50 (an antiferromagnet) in a thin film bilayer with (111) texturing. The intention is to compare experimental results with a micromagnetic calculation that is sufficiently sophisticated to realistically model this polycrystalline ferromagnet/antiferromagnet system. The antiferromagnet thickness dependences of exchange bias and enhanced coercivity were simulated at 10 and 300 K and comparison to experiments revealed reasonable agreement. We also examined the antiferromagnet grain size dependence of exchange bias, owing to its relation to the key issue of uncompensated spin density. Simulation finds a linear relationship between exchange bias and inverse grain size for both thermally stable and thermally fluctuating antiferromagnetic grains, but with different intercepts. Experiment also finds a linear dependence, but the extrapolation to infinite diameter reveals an unexpected negative exchange bias. Our results demonstrate the applicability of this form of micromagnetic modeling for multiple phenomena associated with the exchange bias effect.