Are you spraying sizing on yardage or on cut pieces, pieced blocks?
I seldom use sizing instead of starch but when I do it is on yardage, or large pieces and I iron them. I do the same with starch. I starch ( or size) fabrics before cutting so my cut pieces do not need ironing or pressing. If I am pressing blocks I Press. I use steam if I need to ( like to get many seams in a middle to flatten)