is it possible to set the printer server for custom size paper and envelopes?
By Print Server do you mean a Windows PC sharing the driver or something else?
If just a Windows PC/Server go to Start > Devices and Printers
Right click the printer and go to Printer Properties
In the new window go to Advanced > Printing Defaults
Set whatever defaults you want and it should carry down to the clients at next reconnect.
To apply the change to the server itself go to Start > Devices and Printers then when you right-click go to Printing Preferences instead and do the same.