This is a great question; unfortunately, there is not one definite answer. How big the grids need to be largely depends on the children and their abilities. The more space there is in a grid, the easier the activity should be for the children. Also, the number of children that show up to practice will influence the grid size. The best way to pick a grid size is to have an approximate idea of big the area needs to be; but to be flexible and willing to change the grid size after the activity starts. For example, look at a warm-up game that has everyone dribbling with a ball in an area. We want the kids to have enough space so that they are successful, but if the space is too big then the activity is not as challenging. With a small space the children have to keep their heads up to avoid dribbling into other players. A smaller space also requires the players to stay closer together so they have to interact with each other more often, which will demand the players change their direction and speed. Observe how the players are doing and then adjust the size if necessary. This same principle would apply to all practice activities (no 5v5 full field scrimmages).