The Tablet Area property can be used for that. We use it for calibration (on built-in tablets) but for standalone tablets it can be used to cut the valid area of the tablet down to whatever size. Jason had a patch to add this feature to xsetwacom. see http://old.nabble.com/RFC%3A--PATCH-4-4--Add-ability-to-match-input-aspect-ratio-to-the-current-output-ratio-p32551993.html
(was the first one I found). The same code can be used by GNOME.

Created attachment 213862 [details] [review]
Proposed patch

Patch to add the UI option in the Wacom mapping panel.

Review of attachment 213862 [details] [review]:

::: panels/wacom/cc-wacom-mapping-panel.c
@@ +284,3 @@
 
+	/* Keep ratio checkbox */
+	priv->keep_aspect = gtk_check_button_new_with_label ("Adjust panel area to match aspect ratio");

Problably needs to be a switch instead of a checkbox, and the label will need checking.

Created attachment 214756 [details] [review]
Updated patch

Update patch to use a switch and a different label

Created attachment 214757 [details]
Screenshot

Screenshot of the dialog with patch from comment #4

Jakub mentioned using "Keep aspect ratio (letterbox)" instead.

Created attachment 215712 [details] [review]
Updated patch

Updated label and dialog as  per comment #6 and border width set to 12 as per GNOME HIG.

Created attachment 215713 [details]
Updated screenshot

Screenshot with patch from attachment #215712 [details]

Review of attachment 215712 [details] [review]:

Looks good.

committed