ISSUE: Screenshot of this issue can be seen at :
(1) https://goo.gl/knpdiq 
(2) and also at https://goo.gl/8tDqKe. 

The issue is that the drop down width is not matching the width of top part when a select element is styled by FormDecorator. I am using the latest version i.e. 2017.3 913.

HOW TO REPRODUCE: 
Go to the demo page of FormDecorator at http://demos.telerik.com/aspnet-ajax/formdecorator/examples/overview/defaultcs.aspx.
Expand the only select element on this demo page and you will notice that drop down width is not matching the top part width.

RESOLUTION: 
When I tried to resolve this problem, I came up with a solution that works across all skins and render modes as well as all modern browsers of Chrome, Firefox, Opera, Edge + IE 9,10 and 11. 

The solution involves overriding the method Telerik.Web.UI.RadFormDecorator.prototype._expandHeader.  JavaScript for overriding this method is given in attached file. Just copy this script and paste in anywhere in your aspx page to resolve this issue.