Open burning (for example of wildfires or agricultural fires) produce black carbon-containing aerosol particles of various sizes. Black carbon size is of interest because it affects aerosol optical properties, and may be useful for attribution. We used an extensive dataset of real fires collected as part of the FIREX-AQ campaign in 2019 to associate dependencies in black carbon size with the type of fuel being burnt. We also studied larger particles that are much harder to collect and quantify.