That's right, in the 3800 driver you can choose a "User Defined" size. For the Auto Sheet Feed and Rear Feed you can go up to 17" wide and up to 37.4" long. A lot of users are going for 17" x 25" sheet paper with which they can print 16" x 24" images with an even 1/2" margin all around.
(BTW, I need to update the 3800 FAQ listing above to include the Harman Gloss.)