Full width DropdownButton with adjust dropdown arrow icon in Flutter

Just adding isExpanded:true to the DropdownButton

  Widget example() {
    return new DropdownButton(
          isExpanded: true,
            items: [
              new DropdownMenuItem(child: new Text("Abc")),
              new DropdownMenuItem(child: new Text("Xyz")),
            ],
            hint: new Text("Select City"),
            onChanged: null
        );
  }

Try to add the following in the Column you have...

Column(
  crossAxisAlignment: CrossAxisAlignment.stretch,
  ...
)

You should not need the Expanded widget as that would try to fill the vertical space and not the horizontal (width) space.