my_xlform_screen.rb

class MyXLFormScreen < PM::XLFormScreen
  def form_data
    [
      {
        cells: [
          {
            title: 'my textview',
            name: :my_textview,
            type: :textview,
            cell_class: 'MyXLFormTextViewCell'
          }
        ]
      }
    ]
  end
end

my_xlform_textview_cell.rb

class MyXLFormTextViewCell < XLFormTextViewCell
  def self.formDescriptorCellHeightForRowDescriptor(rowDescriptor)
    100
  end
end