If you want to store different paper sizes in each tray, and have the printer automatically select the correct tray, follow these steps:
For Windows® Users go to Start > Settings > Control Panel > Printers (Printers and Faxes for Windows® XP users).
Right-click on your printer icon and choose Properties.
Choose the Accessories tab and click Auto Detect. In Paper Source Setting click each tray, and then in Paper Size select the appropriate paper size for each tray.
Click Update, and then click Apply.
(Under certain conditions Auto Detect may not be available. If this situation occurs, you can manually add the options you have installed. In Available Options select the option you have installed, click Add and then Apply.)