According to the (conflicting) documentation of TinyMCE, the editor takes on the size of the textarea (or other) element that it replaces. It also says that you can set the size of the editor by specifying { height: '123', width: '123' } in the init method.
I have tried BOTH and still get only one result - the editor resizes itself to (how it remembers is beyond me) what it was the last time a user resized it.