On KDE you can set "keepshape" on "systemsettings", but apparently there's no way to do it in any other DE. The proper way to have "keepshape" working does not seem to work on Debian Wheezy (setting it on xorg.conf.d/50-wacom.conf). The "standard" alternatives are a static xorg.conf, and perhaps figuring hal/udev rules, which I don't know how to do.
Another way to do it is via xsetwacom, explicitly setting the active area along the lines of:...