Try:
<select class="size" name="size" data-setting="size">
<option value="thumbnail">thumbnail</option>
<option value="medium">medium</option>
<option value="medium_large">medium_large</option>
</select>
Try:
<select class="size" name="size" data-setting="size">
<option value="thumbnail">thumbnail</option>
<option value="medium">medium</option>
<option value="medium_large">medium_large</option>
</select>