Bioenergy with Carbon Capture and Storage (BECCS) may be one of the few cost-effective carbon-negative electricity technologies, but little work has focused on design of such systems. BECCS, like other bioenergy facilities, will likely exhibit economies of scale in capital costs, but diseconomies of scale in biomass transportation and supply costs. In this paper we develop a spatially explicit optimization framework to characterize the drivers of optimal sizing for potential BECCS facilities in Illinois. The approach leverages county-level biomass supply data, detailed road transportation networks, existing technology cost estimates, and previous geologic characterizations of long-term CO2 storage. Optimal scales are an order of magnitude larger than proposed scales found in existing literature. Biomass supply, scaling exponents, and technology costs are large drivers of optimal scale, while facility location, pretreatment options, and transportation costs are less important. When choosing between multiple facility locations, economies of scale support a centralized BECCS infrastructure. Deviations from optimal scaled size have little effect on overall systems costs - suggesting that other factors, including regulatory, political, or logistical considerations, may ultimately have a greater influence on plant size than the techno-economic factors we consider here. (C) 2016 Elsevier Ltd. All rights reserved.