Sizing is just a kind of "light weight" stiffener that is put into fabric to give it a nice smooth crisp look.. Ever get what looks like a nice shirt or piece of fabric that you love, but the first time you wash it, it goes limp and is much more open weave than you though and just looks like poo??? that's because the factory sizing has washed out. It's sometimes used by shoddy manufacturers to temporarily make a silk purse out of a sow's ear. It gives body to fabrics without making them stiff.