How to change height of a card in flutter

To modify the width or height of a card you can wrap it in a Container Widget and provide height and/or width properties to it.

Please see below your code wrapped with a container set with height at 500:

Container(
  height: 500,
  child: Card(
    semanticContainer: true,
    clipBehavior: Clip.antiAliasWithSaveLayer,
    child: Image.network(
      'https://placeimg.com/640/480/any', fit: BoxFit.fill,),
    shape: RoundedRectangleBorder(
      borderRadius: BorderRadius.circular(10.0),
    ),
    elevation: 5,
    margin: EdgeInsets.all(10),
  ),
),