Flutter increase height of TextFormField

Just adjust the contentPadding in InputDecoration.

final email = TextFormField(
    keyboardType: TextInputType.emailAddress,
    autofocus: false,
    initialValue: '[email protected]',
    style: new TextStyle(fontWeight: FontWeight.normal, color: Colors.white),
    decoration: InputDecoration(
      hintText: 'Email',
      contentPadding: const EdgeInsets.symmetric(vertical: 25.0, horizontal: 10.0),
      border: OutlineInputBorder(borderRadius: BorderRadius.circular(32.0)),
    ),
  );

You can change the height by changing the minLines value just try this

               TextFormField(
                           keyboardType: TextInputType.multiline,
                           controller: _opTextController,
                           decoration: InputDecoration(
                               isDense: true,
                             border: OutlineInputBorder(
                               borderSide: BorderSide(color: Colors.black)
                             )

                           ),
                           maxLines: 5,
                           minLines: 3,
                           // controller: cpfcontroller,
                         )