The oxalic acid is not for sizing it is a prep treatment to remove excessive buffering in the paper, which can cause problems with tone smoothness in pt/pd.

This may actually hurt the sizing in some papers, but most that we use for pt/pd won't be affected by the oxalic.

There aren't any hard and fast rules on this. Some papers (like Platine) may benefit from a 1% solution brushed on. Other papers (like Fabrianno Uno, or the new Extra White) need to be soaked in a 1 to 2% solution for a few minutes. They will actually fizz and bubble as the carbonate reacts with the acid to produce CO2. I leave them in until the fizzing is gone, and then a little longer to be sure.

There aren't any hard and fast rules on this (did I just say that?). Do it by volume, otherwise, you'll have a fairly strong solution. I use it one shot, because the ox is cheap enough that it doesn't make sense to try to keep it in solution. It may also pick up enough contaminants that you wouldn't want to reuse it again anyway.

Probably the easiest sizing material is gelatine, because you can get it at the store. But you will need a way to harden it. Some people use glyoxol, others use formaldahyde, chrome alum, and other things.

What paper are you planning to use this on? That may help determine what would be the best approach.

FWIW, my attitude is to not make the solution any more complicated than it needs to be. If you don't need to precoat or size, then don't do it, because you risk complicating and compromising the results through other interactions that don't exist without the step. (for example, uneven sizing can cause blotchiness, heavy sizing can result in washoff problems, etc.).