matchParentSize

Gives the child the box's whole extent in place of the extent it prefers, up to an explicit maximumSize where it declares one.

The box is sized to the children that do not match it, so a child declaring this takes whatever size the others settle, and a box whose children all match it asks for its insets alone. Where a maximumSize holds the child below the box, its own align or the box's alignment places it in what that leaves free.