ftpaccess: Please add information on your large Reprap machine design to the wiki. Is it a Mendel Apollo like design, or something different?
When you try to print something larger than the default build volume of the reprap host software, what happens? What error messages are you seeing, and what gets printed? If you can provide more detailed information, we are more likely to be able to help.
You can adjust the various WorkingX (mm) properties (and WorkingY, etc.) in the reprap host software, using the Preferences screen. Without knowing more about your issue, I am not sure if this is what you are asking about.
Jonathan