The following formalism gives the simplest estimation:

Teta (diffraction Limit)=1.22 lambda/D

r= Teta * f

A=Pi* r 2

Where lambda, D , r, f and A denote to be laser wavelength, laser diameter, radius of focused area( spot size), focal length and focal area. however, the success depends of the accuracy of the measurement and how the beam profile is close to TEMoo Gaussian profile ( M2 factor ~ 1.1). Assuming the no lens aberration !

How can I calculate the spot size of a focused laser beam? - ResearchGate. Available from: https://www.researchgate.net/post/How_can_I_calculate_the_spot_size_of_a_focused_laser_beam [accessed Jan 16, 2017].

I have seen this in other places, but why would a larger diameter beam make a smaller spot? It seems that if you expanded the beam first, and then focused it, it would make a smaller spot. But using the same lens, and a smaller beam would somehow make a bigger spot. It seems like a smaller beam should make a smaller spot. Is this some odd quantum physics phenomena?

Can someone explain this in layman terms?