How to add 'marginwidth and marginheight'

Not sure why you want to do this, but the code that generates this can be found in
/fckeditor/editor/_source/classes/fckeditingarea.js (source)

If you don't want to go hacking around in the source ... I know I don't then you can do it after the fact with something like this:

function FCKeditor_OnComplete( editorInstance )
{
var eaIFrame = editorInstance.EditorWindow._FCKEditingArea.IFrame;
eaIFrame.style.marginWidth=0;
eaIFrame.style.marginHeigth=0;