When trying to configure a session printer using the Citrix GPO the following error is shown:

Paper size 1 is not a valid paper size.
Valid paper sizes for this printer are:

The session printer is configured in the Group Policy Management Console on a server with the Citrix XenApp Group Policy Management Experience (x64) installed. In the Group Policy:

- Select User Configuration | Policies | Citrix Policies
- Click New to create a policy
- Enter the policy name then click Next
- Select Printing in the left pane
- Click Add in the Session Printers section
- Click Add
- Enter the print server name as \\SERVER and click Browse
- Select the printer and click OK
- Click OK
- Click OK to complete the Session Printers dialogue

This is when the error will occur.

This is an underlying Point and Print issue. This may be caused by x64 printer drivers not being added under Sharing | Additional Drivers, or incompatibilities between the x86/x64 printer drivers.

1) Check the x64 printer drivers are loaded:
From the print server:

- Right click on the printer and select Properties
- Select the Sharing tab
- Click Additional Drivers
- The x64 checkbox should be checked

2) Test Point and Print
From the XenApp server:

- Select Start | Run
- Enter \\<Print Server Name> and click OK e.g. \\PRINTERS
- Right click on the desired printer and select Connect from the context menu

Once the printer connects and can be printed to from the XenApp server, the Session Printers issue should be resolved.

Ripped off by Citrix: http://support.citrix.com/article/CTX127223