How does intermolecular force vary with the size of the molecule?
Usually, the bigger the molecule, the greater the intermolecular force. This must be qualified by insisting that hydrogen bonding is absent.
The bigger the molecule, the bigger its electron cloud, and hence the greater the opportunity for intermolecular force. Note that hydrogen bonding is known to occur where hydrogen is bound to a strongly electronegative element. And hydrogen bonding is the intermolecular force of the greatest magnitude. Hence the high boiling points of ammonia
Please note that there is a distinction between INTERMOLECULAR force, the force between molecules, and INTRAMOLECULAR force, the forces between atoms within molecules or substances.