Cell Size

Unlike the Horizontal and Vertical Layout Groups, which determine the size of the children either by their Rect Transform component or by scaling them to fit inside the parent's Rect Transform, the Grid Layout Group requires you to specify the width and height of the child objects. You accomplish this by setting the X and Y properties of the Cell Size property. This will automatically apply the specified X and Y sizes to each of the children's Width and Height property of their Rect Transform, respectively.

Due to the Cell Size property and the lack of a Control Child size property, the children are not guaranteed to fit within the parent's Rect Transform. If too many children exist, it is possible they will be drawn outside the parent's Rect Transform. So, if you have a grid filling up dynamically that can change throughout the gameplay and want the grid to always fit within a specific area, you will have to prepare for that scenario of overflow.

The Grid Layout Group allows you to specify both an X and Y Spacing. The X Spacing is the horizontal spacing, and the Y Spacing is the vertical spacing. These values will not be overridden by further property choices, as they can be with the Horizontal and Vertical Layout Groups.