Depends. For instance in the case of the Lennard-Jones potential, which is basically the easiest but still good enough approximation of intermolecular interacti

Depends. For instance in the case of the Lennard-Jones potential, which is basically the easiest but still good enough approximation of intermolecular interactions that we have, you have two fields between ~two molecules : one short-range repulsive, another long-range attractive. This results in a minimum potential at a distance r_min (see graph) that you can arguably think of as the "size" of the (spherical) molecule (at low temperatures, molecules will form "clumps" with that distance between them) : https://en.wikipedia.org/wiki/Lennard-Jones_potential https://en.wikipedia.org/wiki/Lennard-Jones_potential