[![enter image description here][1]][1]

According to _[Wikipedia:Gaussian beam][2]_
the beam waist ($w_0$) and the total angular spread of the beam ($\Theta$)
are related by

$$ \pi w_0 \tan \frac \Theta 2 = \lambda $$

From this formula you see: The wave-nature of light (via its wavelength $\lambda$)
is responsible for the non-zero waist.

That means you would have a beam waist $w_0 = 0$ only for these cases:

* wavelength $\lambda = 0$
  (so that there is no diffraction)
* total angular spread $\Theta = 180°$
  (meaning we have a spherical wave from a point source)


  [1]: https://i.sstatic.net/JLJtu.png
  [2]: https://en.wikipedia.org/wiki/Gaussian_beam#Beam_waist