The title of this thread appears to be wrong, because the parent article says
"But a little experiment that Rayleigh performed in 1890, inspired directly by Franklin's observations, is not nearly as well-known."
Therefore Rayleigh computed the size of molecules in 1890, not in 1870 (in 1870 Rayleigh was young and not known yet for any original research).
While Rayleigh has devised a novel method for determining the size of molecules, it should be noted that the first who has succeeded to determine the size and weight of molecules was Johann Josef Loschmidt, in 1865.
https://en.wikipedia.org/wiki/Johann_Josef_Loschmidt
The publication of the weight and size of air molecules by Loschmidt is one of the most important milestones in the history of physics.
Until that moment in 1865, the theory of atoms revived by Dalton could still be considered as some kind of fictitious model that explained some features of the chemical reactions and of thermodynamics, but which might have been wrong and which would probably be replaced by some better model.
Starting from that moment, the atoms and molecules could be weighed and counted, so their reality was no longer questioned.
The determination by Loschmidt of the size and weight of air molecules was enough to determine the sizes and weights of any other known atoms and molecules, making use of the relative atomic weights that could be determined from chemical reactions and which were already known.
Moreover, a few years later, in 1874, George Johnstone Stoney has used the results of Loschmidt together with the theory of the existence of an elementary electric charge published by Maxwell one year before, in 1873, to compute the value of the elementary electric charge. Some years later, Stoney has given the name "electron" to the elementary electric charge, which has been the source of a very large number of words in modern science and technology, from electronics to hadrons.