A box with a specified size.
If given a child, this widget forces its child to have a specific width and/or height (assuming values are permitted by this widget’s parent). If either the width or height is null, this widget will size itself to match the child’s size in that dimension.
If not given a child, SizedBox will try to size itself as close to the specified height and width as possible given the parent’s constraints. If height or width is null or unspecified, it will be treated as zero.
SizedBox( width: 200.0, height: 300.0, child: const Card(child: Text('Hello World!')), )