<select class="size form-control" id="size" name="size">
<option value="1">first div</option>
<option value="2">second div</option>
<option value="4">third div..</option>
</select>

<div class="collection">random</div>
<div class="collection">text</div>
<div class="collection">inside</div>
<div class="collection">here</div>

<select class="size form-control" id="size" name="size">
  <option value="1">first div</option>
  <option value="2">second div</option>
  <option value="4">third div..</option>
</select>

<div class="collection">Div1</div>
<div class="collection">Div2</div>
<div class="collection">Div3</div>
<div class="collection">Div4</div>

$('div.collection').click(function(){
var index = $('div.collection').index($(this)) + 1;
  $('select').val(index);
});

<div id='parentDiv'>
   <div class="clickMe">1</div>
   <div class="clickMe">2</div>
   <div class="clickMe">3</div>
   <div class="clickMe">4</div>
</div>
<select name="dropDown">
    <option value="1">1</option>
    <option value="2">2</option>
    <option value="3">3</option>
    <option value="4">4</option>
</select>

<style>
.clickMe{
  clear:both;
  width:20px;
  height:20px;
  border:1px solid black;
  margin: 20px;
}
</style>

<script>
$('#parentDiv').on('click','.clickMe',function(){
  console.log($(this).index());
      var index= $(this).index();
      $('select[name=dropDown] option:eq('+index+')').attr('selected', 'selected');
});
</script>