5
What is the problem? It shows a multi selection box instead of single selection box? Is not that the idea of searching?
If you want to make it single selection then go to search.php and change line 87:
case "select":
$options = $fields[$i]->getVar('field_options');
$size = MIN( count($options), 10 );//change 10 to the max-size you want for the box;
$element = new XoopsFormSelect($fields[$i]->getVar('field_title'), $fields[$i]->getVar('field_name'), null, $size, false);//changed true to false
asort($options);
$element->addOptionArray($options);
$searchform->addElement($element);
unset($element);
break;
But this will have effect in all select boxes you have.