Open a Terminal window and type dtpad ~/.Xdefaults.
When the file is displayed, it may be empty or there may be text already in the file. In either case, add the following four lines of text to the file:
When you type in these lines, replace the default numbers at the end of each line with the new display area size you want.
Choose Save from the File menu.
Choose Close from the File menu.
The change will not take effect until you exit the desktop and log back in. If you should want to return to the default display area size, type the default sizes back in or just remove the four lines you added, and then exit and restart the desktop.