Like all 24 inch printers (except for one odd older HP which had a 17 inch tray, although it would take 24 inch rolls), the 7900 has no paper tray. The only way to print letter sized sheets is to load them MANUALLY, one sheet at a time. It's probably worth having a 3800 or a Canon iPF 5100 around for letter sized printing (both are 17 inch printers that take large ink cartridges (so running costs are more reasonable than smaller printers) and have paper trays. Of course, your other option is to use 11 inch roll paper on the 7900 when you want to print letter size - it'll print and cut that just fine. I have an iPF 5000 (the older version of the 5100 I recommended), and I print almost everything on 17 inch rolls, but I have run a few boxes of letter size through the tray, and it works fine - you can get a 25 sheet pack of thick art paper in without a problem...
-Dan