The mean free path l for a gas molecule depends upon diameter, d of...

The mean free path is an average distance over which a moving particle substantially changes its direction or energy, typically as a result of one or more successive collisions with other particles.

The mean free path for a gas molecule depends upon the diameter, $d$ of the molecule as: $\lambda = \dfrac{1}{\sqrt{2}\pi{d}^{2}n} ∝ \dfrac{1}{{d}^{2}}$,

$\lambda =$ the mean free path
$d =$ the effective diameter of the molecule
$n =$the number density of molecules