It's difficult to determine the cause of this odd behavior without being able to see your code. Where do you set the "EnableSingleInputRendering" to false? You should do this in the .aspx markup and not in the code behind, so that it will apply even with a postback. Also, how are you setting your width property? Are you doing this in the .aspx, or by using CSS or in code behind?
I would also suggest that if you continue to have problems setting and maintaining the correct width, you should open a support ticket
with Telerik and provide them with your complete code, including any custom CSS that you are using. This will help to expedite a solution for you.
There have been recent changes to the method in which RadInput controls are rendered, so you may want to download the latest internal build
(currently the Telerik.Web.UI_2012_1_327_Dev) and try that one without setting the "EnableSingleInputRendering" to false (which does not use the newly implemented rendering approach which will eventually become the standard approach).