Not a simple question and one which probably needs the physicist people to offer comment. 'Size' of molecules and atoms (which, I might add, is out of my routine ambit) is a bit rubbery due to the notion of electrons having both mass and wave properties. Probably, at the air temperatures for which you most likely are interested, such notions won't be significant.
The following
paper might give you some food for thought .. albeit at the expense of needing to skip over a few of the sums ....