Please, note that I do not recommend setting the AutoResizeHeight property to "true" because RadEditor the code that sizes the content uses the toolbar height in the calculations and when the toolbar hidden a side effect appears in Firefox. For that reason it is better to not set the AutoResizeHeight property.
The example is based on this live demo: http://demos.telerik.com/aspnet-ajax/editor/examples/editorastextbox/defaultcs.aspx