As far as I know a region can be either ContentControl or ItemsControl. If your region is ContentControl, then try setting the HorizontalContentAlignment and VerticalContentAlignment to Stretch. If your region is ItemsControl, then use ItemContainerStyle to set HorizontalAlignment and VerticalAlignment to Stretch. Give it a try and let me know how this works for you. If you still experience the same issue, could you please share more details about your application. It would be best if you could send us a sample project.
the Telerik team