The content area size is exceeding RadEditor's height in IE11 with its default document mode "Edge". Currently the issue could be worked around by setting the following meta tag.

      <meta http-equiv="x-ua-compatible" content="IE=10">
The approach is described on the following article:

http://msdn.microsoft.com/en-us/library/ie/jj676915%28v=vs.85%29.aspx