Thanks Drew. Jidanni, try the following two advices if interest.
Probably this would be what you want `text-scale-adjust' to do.
The first one tweaks the `window-width' function so to return
a value based on the text scaling, and the second one makes the
`text-scale-adjust' function redisplay an emacs-w3m page. It's
only a quick hack, so I don't want to imprement it in emacs-w3m.
(defadvice w3m-redisplay-this-page (around adjust-window-width activate)
"Adjust window-width value according to `face-remapping-alist'."
(current-buffer)
(let ((height (ignore-errors
(cadr (assq :height
(assq 'default face-remapping-alist))))))
(if height
(let ((ofn (symbol-function 'window-width)))
(fset 'window-width (lambda (&rest args)
(floor (/ (funcall ofn) height))))
(unwind-protect
ad-do-it
(fset 'window-width ofn)))
ad-do-it)))
(defadvice text-scale-adjust (after redisplay-w3m-page activate)
"Redisplay w3m page after scaling text."
(when (eq major-mode 'w3m-mode)
(let ((w3m-message-silent t))
(w3m-redisplay-this-page))))
