In the print preview's upper right corner is a button "Settings..." that opens yEd's print settings dialog. Open said dialog and make sure to use the default settings (simply click the dialog's "Reset" button and confirm your decision with "Ok" when asked for it). Close the settings dialog by clicking the dialog's "Ok" button.
In the print preview's upper left corner is a button "Page..." that opens your printer's paper/print settings dialog. Open said dialog and choose the correct paper size. Close the settings dialog by clicking the dialog's "Ok" button.
Finally, print your diagram using the preview's "Print..." button.