Select the envelope size in the settings of the application ("Page Setup" in many applications) and then perform the following steps.
Select the envelope size from the "Paper Size" menu on the page setup screen and click the [OK] button.
In Mac OS 9, select the envelope size from the "Paper Select" menu on the page setup screen.
Select [Paper Feed] in the print settings window.
Select [Bypass Tray (Envelope)] from the "All pages from" menu.
In Mac OS 9, select [Bypass Tray (Envelope)] from the "All pages from" menu of "Paper Source".
Set the paper type of the bypass tray to [Envelope] and load an envelope into the bypass tray.