Not at all.
The size of an atom was computed for the first time 40 years earlier, in 1865, by Johann Josef Loschmidt, who was an Austrian scientist.
Loschmidt had determined the value of what today is called "Avogadro's number", despite the fact that Avogadro had no idea about how one could find the value at that number. The contribution of Avogadro had been the law that ideal gases have the same number of molecules per volume when the pressure and the temperature are the same, but he did not know how that number might be determined.
Based on the results of Loschmidt, which were also used by Maxwell in his works on statistical physics, a few years later, in 1874, George Johnstone Stoney computed the value of the elementary electric charge, which he later named "electron".
Loschmidt was the one who first measured and weighed the atoms, establishing their existence beyond any reasonable doubt. Before him, many believed that the hypothesis of Dalton about the existence of atoms is just a convenient theory for explaining the laws of chemistry, but some other better explanation might be discovered in the future.
Einstein published an explanation of the Brownian motion, which verified that everything fits in the known theories and it matches the expected values. It did not contain any novel number or law, but it was a useful confirmation of the current theories.
In my opinion, the greatest achievement of Einstein has been the paper published by him in which he revealed the existence of the stimulated emission of electromagnetic waves by hot bodies, besides their well known absorption and spontaneous emission.
This paper had tremendous practical applications, leading to the discovery of masers and lasers, without which the modern technology would have never existed.
While Einstein's theory of gravitation was more intellectually challenging, its practical applications are minor, almost negligible. Moreover, even after more than a century we do not know exactly how adequate it is for describing gravitation.
What is certain is that Einstein's theory of gravitation cannot be anything else but an approximation, as it is based on averaging the distribution of matter in space, without taking into account the concentration of matter into elementary particles.
Another contribution of Einstein that is more important than his most frequently cited works is his support for Satyendra Nath Bose, which lead to the recognition of the Bose-Einstein statistics, which together with the Fermi-Dirac statistics plays an essential role in quantum physics.