The mean free path $\lambda$ of a gas molecule of diameter $d$ at number density $n$ is approximately:

The mean free path is the average distance a molecule travels between collisions. Standard kinetic-theory derivation gives: $\lambda = \dfrac{1}{\sqrt{2}\pi d^2 n}$ where $d$ is the molecular diameter and $n$ is the number density. Note the $d^2$ (cross-sectional area), $\sqrt{2}$ factor from accounting for the relative motion of two molecules colliding (both move, not just one), and the inverse-density dependence (more molecules around means more collisions, shorter path). At STP, $\lambda$ for air is about 70 nm — small but huge compared to molecular size (a few Å).