Older Wacom tablets have a 4:3 or 5:4 aspect ratio, while modern monitors have 16:9 or 16:10, this leads to a distortion of the drawing area. One can get used to it, but one can also easily fix it by using xinput, by issuing a command like this:
xinput set-int-prop "Wacom Intuos2 6x8" "Wacom Tablet Area" 32 0 1770 20320 14470
This will change the active area on the tablet to the last four numbers on the line, which are left, top, right and bottom, different tablets will need different numbers, just calculate them from the default ones. The 32 in that line is just there to tell xinput that we are providing a 32bit integer, it is not a configuration parameter for the Wacom device itself.
The name of the tablet varies, to find out the correct one, one uses:
There are also plenty of other configuration parameters that one can tweak, a full list is available with:
xinput list-props "Wacom Intuos2 6x8"
This list also includes the current values of all the properties, so if you don't know what the area of your Wacom is, you can use it to look it up.
Note that a Wacom tablet will normally show up as multiple devices, your pen is one, your eraser is another and the mouse is yet another, so you might want to change the active area for all those devices, not just one. Making changes with xinput is not persistent, so after every restart of Xorg, it will be back to its default values.
An alternative way to accomplish this is described in the Ubuntu Documentation, which works by using the KeepShape option in xorg.conf.