customizing size of emacs windows in twm, depending on the vnc session

I connect to the same Unix system (that runs tab windows manager, twm) via vnc. I connect from a laptop with a small screen and from a workstations with a large screen. I have started two different vnc sessions with two different --geometry setting because of the different screens (I like to use the full screen in both cases). The two sessions share the same emacs initialisation file. The size of emacs window I like on the workstation screen is too large for fitting the laptop screen. The part of the configuration file where I set the size is like the following: ;; set font (cond ((string-equal system-type "windows-nt") ; Microsoft Windows (progn nil ) ) ((string-equal system-type "darwin") ; Mac OS X (progn (add-to-list 'default-frame-alist '(font . "Monaco-20")) ) ) ((string-equal system-type "gnu/linux") ; linux (progn (add-to-list 'default-frame-alist '(font . "DejaVu Sans Mono-20")) ) ) ) (set-face-font 'menu "-*-*-*-*-*-*-20-*-*-*-*-*-*-*") (set-face-font 'default "-*-*-*-*-*-*-20-*-*-*-*-*-*-*") (add-to-list 'default-frame-alist '(height . 60)) (add-to-list 'default-frame-alist '(width . 120)) I think I should set the size of the emacs window relatively to the size of the screen but I don't know how to do it in twm, or maybe I should put a switch depending on the vnc session. does anybody know how to do that?