MSND-85 Set the initial size of the <textarea>

This results in the CKEditor space being roughly the same size raw <textarea> and so we don't end up with double scroll bars.

git-svn-id: https://source.sakaiproject.org/svn/mailsender/trunk@125704 66ffb92e-73f9-0310-93c1-f5514f145a0a