flutter how to give height to the childrens of GridView.Builder

You want to use the childAspectRatio property of the SliverGridDelegate preferably with MediaQuery:

class HomePage extends StatelessWidget {
  final List<String> items = <String>[
    "Item 1",
    "Item 2",
    "Item 3",
    "Item 4",
    "Item 5",
    "Item 6",
  ];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Home')),
      body: GridView.builder(
        gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
          crossAxisCount: 2,
          childAspectRatio: MediaQuery.of(context).size.width /
              (MediaQuery.of(context).size.height / 4),
        ),
        itemCount: items.length,
        itemBuilder: (context, index) {
          return GridTile(child: Text(items[index]));
        },
      ),
    );
  }
}