J.J Thompson's Cathode Ray Experiment

[AlphaK]

When Thompson conducted his cathode ray experiment, how did he know that the negatively charged electrons were so small? He knew that they were deflected in measurable ways by electric and magnetic fields, so he concluded that they must have a very small mass to charge (m/e) ratio. But that could also have been explained by the largeness of charge?

 

[AlchemistK]

See Millikan's oil drop experiment. It was only after that experiment that the exact charge and size of the electron was found out.