I am interested in knowing exactly what "glue size" is, as well as applications for using it.
When machining a low density wood (softwoods or hardwoods) or a medium to dense hardwood with substantial amounts of reaction wood, wood fibers may be so weak that they are pushed over instead of cut off. This is especially obvious when sanding--the wood develops very fine "peach fuzz." Although this may be the effect of dull sandpaper or a dull knife, it can often be remedied with glue size. Glue size is used to stiffen and strengthen wood fibers so that they can easily be sanded or lightly planed off.
Glue size is also applied to the porous edges of particle and fiber boards to prevent over-absorption of glues and finishes. This helps fill the grain and give the wood more even coloration and surface quality.
Because many of the materials used for glue size become soft and will flow when heated, it is necessary to avoid high temperatures caused by the friction of aggressive sanding. It is also important to keep the temperature controlled during curing of the finishes.
Most of the time, commercial glue size formulas are better than homemade. Glue size is most commonly made of a watered-down, water-based, PVA adhesive. If you do plan to make your own, try something on the order of 10 parts (or a little less) water to one part white glue. Keep experimenting if this doesn't work out right (it will vary with species).