how to set showModalBottomSheet to full height?

[Update]

In showModalBottomSheet(...) set the property isScrollControlled:true.

It will make bottomSheet to full height.

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[Original answer]

You can Implement the FullScreenDialog instead.

Flutter Gallery app has an example of FullScreenDialog

You can open your Dialog using below code:

Navigator.of(context).push(new MaterialPageRoute<Null>(
      builder: (BuildContext context) {
        return Dialog();
      },
    fullscreenDialog: true
  ));