double in size means: This refers to the last rising proofing prior to bread being baked. It is visual and can be guessed. A template is used by some bakers to indicate how high the bread should be above the edge of the pan. You can find a recipe guide or a formula guide. “34 proof=almost twice as big” and “full proof=almost twice the size.” Bake if there is any slight mark on the loaf. (in Cooking Dictionary)