Quantum Electronics: Minimizing Beam Volume in Gas Cell

[physics0o]

Homework Statement

In many laser applications the volume occupies by a laser beam in a gaiven crystal should be minimized. so how i can define that volume for a gas cell(n=1,00) that is 4cm long, using a laser wavelength of 633nm and placing the focus in the middle of the cell.

Homework Equations

The Attempt at a Solution

 

[physics0o]

Is this about Gaussion Beam?
I am not sure what the question is and what i have to do.
The volume is related to waist, isn't it?
so could someone kind enough to give me some hints that how I can start?

Thank you!