You can change the paper size for documents which are printed using DocuWare Printer in the registry. The default paper size is ISO A4.

Download the attached "PaperID.reg" and execute it on the client, in order to create a new key in the registry.
Now edit the dword PaperID to the size you want:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Print\Printers\DocuWare Printer\PrinterDriverData

All available sizes are listed under
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Print\Printers\DocuWare Printer\PrinterDriverData\Papers\

Every size has an own PaperID. Adapt the value of the PaperID in PrinterDriverData to the one you prefer.
For example
"00000001" for Letter size
"00000009" for A4.
After making the appropriate change, be sure to recycle the Print Spooler, Desktop Service, and Desktop App.

NOTE: If you are testing these configuration please do not use Windows Test Page from within the Printing Preferences. Try using any application to test these changes. 

File for download:
- PaperID.reg