Vinegar and water is the time-honored method. The sizing is a good idea, but sizing and sarch add to the fabric. I like Mary Ellen's Best Pressed. I get it a gallon at a time. When I spray it on crinkled fabric, I can watch the fabric relax. It adds body without stiffness or white flakes. JoAnns has it in various scents and scent free.
Good luck and thanks for your kindness.