Measuring aerosol size distributions with precision is important to understand the transmission of infectious diseases causing pneumonia-like illnesses, including Legionnaire’s disease and COVID-19. Accurate size measurement can help guide the identification of risk mitigation strategies, a challenging task as the size of pathogen carrying particles can change due to evaporation. Using oil and water-based particles, we compare four measurement instruments that provide aerosol size distributions relying on different measurement principles; phase doppler anemometry, light scattering, electrical mobility and aerodynamic impaction. The size distributions obtained from the different measurement instruments are consistent for oil-based particles but differ significantly for particles experiencing evaporation. Experiments were conducted to quantify the differences in measurement caused by evaporation as well as the measurement losses. The size range covered by the four instruments is from 0.025 to 10 µm. Finally, an approach based on the complete drying of a salt solution is presented to obtain the size distribution at the source of the aerosol generation. Understanding of the infectious dose and the pathogen concentration at the source, the most dangerous size ranges of the aerosols can thus be determined, and actions can be taken to reduce the propagation of these particles.