I don't know about this molecular size argument. Argon gas is in the form of single Argon atoms, while Nitrogen gas is in the form of N2 molecules, and without looking it up because I don't have the reference at home I would be surprised if the N2 molecule wasn't larger, or at least longer, than the Ar atom.
I also would be surprised if either gas doesn't leak out over time, but purging the scope with a dry gas like nitrogen or argon will ensure that it starts out dry and will probably stay dry for some time. Certainly if you open your scope, whatever nitrogen or argon is in there will instantly depart, Mr. Atkinson's statements to the contrary.
