Why a big sheet changes what the whole body does.

Paper size is not a detail of presentation. It decides which muscles are involved.

Give a young child a small sheet and the work happens at the wrist and fingers, tightly, in the middle, with a wide margin of untouched white. Give the same child a sheet large enough that they have to lean across it or stand up and the work happens at the shoulder. Lines get longer and more confident, marks get bolder, the whole surface gets used, and the child moves while working. It is the cheapest and most reliable intervention available and it takes no skill from the adult beyond a willingness to hand over more paper than seems reasonable.

The same logic applies to the surface. Working flat on the floor lets a child crawl around the sheet, approach it from all four sides, and lean their weight into a mark. Working upright at an easel or at paper taped to a wall changes it again: gravity pulls wet colour downwards, which is a discovery in itself, and the arm swings freely because it is not resting on anything. Neither is better. What is useful is that both are available at different times, because each one produces work the other cannot.

Adults tend to default to small and flat and seated because it is tidy and contained. Understandable, and worth breaking regularly. Roll out a long strip and let it run the length of the hallway. Tape a sheet to the side of a cupboard. Put a board on the floor and let them kneel. What you are managing here is not the picture, it is the amount of room the body has, and children who are given room use it. The work that comes off a large surface is almost always looser and more alive than the work that comes off a small one, and that is the tool doing it, not talent.