$field = 
  Rose::HTML::Form::Field::SelectBox->new(
    name     => 'whatever', 
    multiple => 1,
    options  => [ 1 => 'One', 2 => 'Two', 3 => 'Three' ],
    size     => 3);

$field->input_value([ 1, 3 ]);

print $field->html;