I have achieved just that by manually altering the following code in \core\cfdefs\cfdef_standard.php:

function cfdef_input_textbox($p_field_def, $t_custom_field_value) {
$t_control_size = 80;
$t_max_length = 255;

if( 0 < $p_field_def['length_max'] ) {
    $t_max_length = $p_field_def['length_max'];
    if ( 80 > $p_field_def['length_max'] ) {
        $t_control_size = $p_field_def['length_max'];
    }
}

echo '<input ', helper_get_tab_index(), ' type="text" name="custom_field_' . $p_field_def['id'] . '" size="' . $t_control_size . '"  maxlength="' . $t_max_length . '"' .
' value="' . string_attribute( $t_custom_field_value ) .'"></input>';

}

Included in part 1 of duplicate 0004076