is a light beam round

I asked myself that question. Here is an excerpted answer:
​
A light beam is light which propagates essentially in one direction (directional light) and at the same time has a limited spatial extension in directions perpendicular to its beam direction. Often, it has a roughly circular cross-section but no precise spatial limitation.

A light beam with circular cross-section may be characterized with a beam radius which generally changes during propagation. Over long enough distances, it must increase due to diffraction, since light is a wave phenomenon (→ wave optics). However, the beam radius may initially decrease before reaching a beam focus (or beam waist) where it has its minimum value. The increase in beam radius in the far field is associated with the beam divergence.

A light beam can actually have two different focus positions e.g. for x and y direction; this is called astigmatism.
https://www.rp-photonics.com/light_beams.html#:~:text=A%20light%20beam%20is%20light,but%20no%20precise%20spatial%20limitation.​