I emailed your question to my daughter who a chemistry major, but who is in China right now. Following is her response:
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yes, they get bigger. all periodic trends are based on these two things
1. shielding effect
2. successive energy levels
both of these things affect the size of hte molecule. the more energy levels, the larger it is. the more shielding effect between electrons in the orbitals, the more they repel (like two magnets and you put the south ends together) and the larger hte atom will be. when atoms are fatter, they get in each others way more. kinetic molecular theory states that atoms or molecules hitting each other with the right energy in teh right spot will react. intermolecular forces is basically a specific case of this. except, they don't react to form new compounds, so they don't have to hit in a certain location. they just get in each others way. imagine a school where everyone is exactly the same, physically. if everyone's skinny and there's 2k of them, then they won't get in each others way too much. so there are less intermolecular forces. where as if you have say 2k morbidly obese kids in the same school, it would be really hard for any one kid to get around. thus, there are stronger IMF forces.
