Select the envelope size in the settings of the application ("Page Setup" in many applications) and then perform the following steps.
Click the [Paper] tab and select the envelope size from the "Paper Size" menu.
Select [Bypass Tray] from the "Paper Source" menu of "Paper Selection".
Select [Envelope] from the "Paper Type" menu.
Set the paper type of the bypass tray to [Envelope] and load an envelope into the bypass tray.