Here is a simple way to add an hook to both the activation and deactivation of ecb:
(add-hook 'ecb-deactivate-hook
(lambda () (modify-all-frames-parameters '((width . 80)))))
;; resize the ECB window to be default (order matters here)
(add-hook 'ecb-activate-hook (lambda () (ecb-redraw-layout)))
(add-hook 'ecb-activate-hook
(lambda () (modify-all-frames-parameters '((width . 120)))))
It is like an hack, but it works