The mean free path of molecules of a gas, (radius ‘r’) is inversely proportional to:

The mean free path $\lambda$ is given by:
$$\lambda =\frac{1}{\sqrt{2}\pi d^{2}n}$$
where $d$ is the diameter of the molecule ($d=2r$) and $n$ is the number of molecules per unit volume.
$$\lambda \propto \frac{1}{d^2}⟹\lambda \propto \frac{1}{(2r{)}^2}⟹\lambda \propto \frac{1}{r^2}$$
Thus, $\lambda$ is inversely proportional to $r^2$.