I am pretty much in the same situation. No idea how to edit the forms using decorators and such.
Also seems to be no way of changing the size of a textarea when using Zend_Form_Element_Textarea where as using Zend_Form->addElement you can add rows and cols in the options array.
EDIT: Actually you can change the size of the textarea by adding
Code:
->setAttrib('rows','10')
->setAttrib('cols','50')