Ethanol produced from cellulosic feedstocks has garnered significant interest for greenhouse gas abatement and energy security promotion. One outstanding question in the development of a mature cellulosic ethanol industry is the optimal scale of biorefining activities. This question is important for companies and entrepreneurs seeking to construct and operate cellulosic ethanol biorefineries as it determines the size of investment needed and the amount of feedstock for which they must contract. The question also has important implications for the nature and location of lifecycle environmental impacts from cellulosic ethanol. We use an optimization framework similar to previous studies, but add richer details by treating many of these critical parameters as random variables and incorporating a stochastic sub-model for land conversion. We then use Monte Carlo simulation to obtain a probability distribution for the optimal scale of a biorefinery using a fermentation process and miscanthus feedstock. We find a bimodal distribution with a high peak at around 10–30 MMgal yr−1 (representing circumstances where a relatively low percentage of farmers elect to participate in miscanthus cultivation) and a lower and flatter peak between 150 and 250 MMgal yr−1 (representing more typically assumed land-conversion conditions). This distribution leads to useful insights; in particular, the asymmetry of the distribution—with significantly more mass on the low side—indicates that developers of cellulosic ethanol biorefineries may wish to exercise caution in scale-up.