Oxalic is not really a sizing agent as much as a buffer neutralizer for papers. Pt/Pd is an acidic process, and the trend toward archival (i.e. pH+) papers means that paper makers now incorporate significant amounts of carbonate buffer in their papers to create a non-acidic paper that doesn't age (thus yellow) as quickly. Unfortunately these same buffers are inimical to the pt/pd sensitizer blend (pH around 3 if memory serves) and give weak, anemic prints unless the buffer is neutralized in this oxalic acid solution prior to coating. This treatment makes many papers that absolutely suck as pt/pd papers as delivered actually work quite well after treatment in an oxalic acid bath. A great example is the new Fabriano Extra White. Coat it straight out of the package and you have a really lousy print. Soak it in 1% oxalic acid for five minutes, dry and then coat it and the paper really looks great.
Sizing, on the other hand, is actually a way of filling the pore space of the paper with a filler - either starch or gelatin - to give the paper more body and make it less absorbent. Most of us size papers that we intend to use for gum layers to prevent the gum from staining our highlight areas.
Most good pt/pd papers have enough internal sizing that it would be redundant to size them again. On the other hand, if you wanted to print on some unusual paper like newsprint, both sizing and oxalic would probably be necessary to get a decent print.
Hope this helps.