Question

What do we assume about the volume of the actual molecules themselves in a sample of gas, compared to the bulk volume of the gas overall? Why?

Answer 1

One of the assumptions made in the Kinetic Molecular Theory is that the molecules have zero volume. Neglecting the volume term (and it is small) allows to to work with a much simpler equation in the general gas law of PV = nRT. There are other assumptions, too. You can read more about it here.
http://www.chm.davidson.edu/vce/kineticmoleculartheory/basicconcepts.html