Gordon,

It always assumed that an airplane size (scale, whatever) is known when discussing wing loading. It is like discussing sex without saying if the object of discussion was a man or woman.

It is incorrect to think of the number as a "volume" as indeed the volume inside of an airplane is quite independent of the wing area.

The response of a wing to air has everything to do with Reynolds number - if it doesn't then all of the work I have done over a lifetime matching wind tunnel Reynolds numbers to data goodness has been pretty well wasted!!

I agree with Dick that in terms of model work ounces per sq ft is a good rule of thumb to use but again it does need a airplane size to go with it. Terms like .40 or .60 powered, hand launched gliders, parkfliers, pattern, scale, giant scale, etc. all immediately bring to the mind a relative size and wing loading that works nicely.