Annie, is not a sizing per se, but a pre bath. You immerse the entire sheet in the solution or you can just skim the sheet over the solution, depending on the paper.
I use a 2% solution by weight, 2 grams in 100 ml of water, I think Clay uses a 1% solution, do some testing and see what is best for your paper.
I save the oxalic acid solution, but if you dont use it often enough it starts to grow mold, so essentially I would say it is a one shot deal.
As before, you will have to do some testing and determine how long you have to immerse the paper, I usually do 5 minutes, but Socorro has a lot of alkaline sizing, I can even smell the stuff after I am done.
Good luck and have fun.