flutter 부모크기에 반응하는 TextFormField 설정

...
Expanded( //expand parent
    child: Column(
        children: [
            Expanded( //expand itself
                child: TextFormField(
                controller: rstTxtController,
...
                minLines: null, //min to null
                maxLines: 999, //max to max
                keyboardType: TextInputType.multiline,
            ),
...