What I can suggest you would be to check the CustomHeaderTemplate
example from our XAML SDK repository. Using the same approach you would be able to apply a Template with different Width depending on the current header. You can use an IValueConverter as well and use single template and change the Width. For your convenience I have modified the same example in order to demonstrate the exact approach.
Please check it and let me know if it helps.
Check out Telerik Analytics, the service which allows developers to discover app usage patterns, analyze user data, log exceptions, solve problems and profile application performance at run time. Watch the videos
and start improving your app based on facts, not hunches.