Override to control the size of the container for the children.
By default, the flow will be as large as possible. If this function returns a size that does not respect the given constraints, the size will be adjusted to be as close to the returned size as possible while still respecting the constraints.
If this function depends on information other than the given constraints, override shouldRelayout to indicate when when the container should relayout.
Size getSize(BoxConstraints constraints) => constraints.biggest;