Abstract Refractory black carbon (rBC), a primary aerosol material, is radiatively significant and a uniquely useful tracer of aerosol processes. Here we present analyses of open‐burning generated rBC size distributions observed both from a research aircraft and from ground‐based sampling. The aircraft sampled fresh smoke generated by 14 different western wildfires and 80 agricultural/prescribed fires, providing sufficient data for statistical analysis of rBC size. The ground‐based observations represent a narrower range of smoke sources, but provide a constraint on contributions to total rBC concentrations from larger particles than those quantified from the aircraft. The range of rBC mass median diameter observed in log‐normal fits was 0.11–0.2 μm volume‐equivalent diameter, with geometric standard deviations ranging from 1.25 to 1.5. rBC size showed a dependence on fuel type. We hypothesize that fuel density and structure significantly influence rBC size in open burning emissions via their influence on the physical conditions of open burning.