Are your Page Layout settings correct? You can set the size of paper to A4, Letter, or Legal. This is found in Options / Page Layout.
What version are you using? We had a similar problem with the packing slip. We filed a support ticket. It turned out to be a glitch in the program. They fixed it and issued an update and sent us the link.
Might have something to do with your Custom Field. Have you tried printing one without the custom field?