[Update]
In showModalBottomSheet(...)
set the property isScrollControlled:true
.
It will make bottomSheet to full height.
[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
));
Check this blog post too for more:
Hope it will help you.