I had the same problem at one time, except I was able to see the bad print size in the preview window.
First, I found that I only had to set the Document Size on the Paper/Quality tab, and that I could leave the Resizing Option on the Features tab set to Actual Size. I had been trying to keep these two in sync, but the Resizing Options drop-down kept losing the paper sizes I had set up, and would fall back to A4.
Second, you can change the default paper size and other settings in Windows: Start / Printers and Faxes / right-click on Z3100 and select Printing Preferences...
- On Paper/Quality tab, under Document Size, select your paper size. As long as you're here, check Show Preview Before Printing.
- On Features tab, make sure "Actual size" is selected under Resizing Options.
- Again, as long as you're here, go to the Color tab and select Application Managed Colors.
- Click OK.