I tried setting size to 2 or 1, it did not make any difference.
I also read Height of an HTML select box (dropdown)
Is there no other option apart from using the plugin - http://programmingdrunk.com/current-projects/dropdownReplacement/?
Code from Comment:
<select class="form-control" data-placeholder="color" chosen ng-model="obj.color" ng-options=" color.name for color in colors" size=1>
<option value=""></option>