How to configure Wacom tablet area on Ubuntu 14.04?

I do not think that there is a graphical configuration interface. Nevertheless, you can do it via scripts...

Find the correct name of your device/input:

[romano:~/personal/varios] 1 % xsetwacom --list devices
Wacom Intuos PT S Finger touch      id: 12  type: TOUCH     
Wacom Intuos PT S Finger pad        id: 13  type: PAD       
Wacom Intuos PT S Pen stylus        id: 14  type: STYLUS    
Wacom Intuos PT S Pen eraser        id: 15  type: ERASER 

And now you can reduce for example to the top left corner:

xsetwacom --set "Wacom Intuos PT S Pen stylus" MapToOutput 500x400+0+0

or in a rectangle in the center (more or less):

xsetwacom --set "Wacom Intuos PT S Pen stylus" MapToOutput 500x400+500+500 

Go back by telling the full resolution:

xsetwacom --set "Wacom Intuos PT S Pen stylus" MapToOutput 1980x1080+0+0

There is a way to make the thing permanent, but I never tried it --- if you need it, the Arch page (as ever) is full of interesting info.

With MapToOutput you can specify a specific screen (e.g.: VGA) or screen size that can let you map the tablet area to two monitors.

If what you need is to force proportions or reduce the tablet area, then you can use the xsetwacom parameter Area:

$ xsetwacom list parameters | grep Area
Area             - Valid tablet area in device coordinates. 
ResetArea        - Resets the bounding coordinates to default in tablet units. 

For example, in my Bamboo tablet, I first reset the area and get its original device coordinates:

$ xsetwacom --set "Wacom Bamboo 2FG 4x5 Pen stylus" ResetArea
$ xsetwacom --get "Wacom Bamboo 2FG 4x5 Pen stylus" Area
0 0 14720 9200

Then I set them to get a smaller tablet area (1/4th of the tablet area):

$ xsetwacom --set "Wacom Bamboo 2FG 4x5 Pen stylus" Area 0 4600 7360 9200

so I can achieve that the bottom-left rectangle maps to the whole screen size.
This way, I can reach that whole area with less hand movement.

The excellent arch wiki recommends that you use the name of the display directly into the MapToOutput parameter. You can obtain the display names using the xrandr command:

xrandr
xsetwacom --set 11 MapToOutput DVI-0

This way you don't need to mess up with any numbers. Please also mind that each time you set the above parameter, the orientation needs to be set again using the Rotate parameter.

In my particular setup I have two displays and I made a small script to switch the wacom between the displays. Then I assigned the script to a custom keyboard shortcut:

#!/bin/bash
WFILE=/tmp/wacom
if [ -f $WFILE ]; then
  WACOM="`cat /tmp/wacom`"
else
  WACOM="DVI-1"
fi

if [ "$WACOM" = "DVI-0" ]; then
  WACOM="DVI-1"
else
  WACOM="DVI-0"
fi

echo "$WACOM" > $WFILE
xsetwacom --set 11 MapToOutput "$WACOM"
xsetwacom --set 11 Rotate "half"