I used Photoshop Elements to do the math. When I create a blank canvas at 4088 X 1784 pixels, then constrain proportions and enter a height of 8.3 inches, it gives me a width of 19.019 inches. This may be why you aren't getting the width you expect.
More math - equal to 4088 X 1784 pixels:
19.019 X 8.3 inches
18.332 X 8.0 inches
13.627 X 5.947 inches
This is, however, academic. I don't know how to custom crop to these dimensions and force a print of the entire image. I don't know if iPhoto can do it. If you don't have Photoshop Elements 4 you might want to look into it, since your printer is so advanced. Printing custom sizes is much easier in PE 4, and it's a great photo editing application.
My Epson is a Stylus Photo RX620, and I'm very happy with it. It can copy, scan, print, and read a variety of media cards. With the right paper, I can't tell the difference between drugstore prints and my own borderless 4 X 6 prints. Your panorama feature sounds really cool. (I'd love to try it out on one I made of a wind farm in west Texas.) I suppose it does take a lot of playing with to get the feel for it.
Enjoy.