The maximum print size on a 3800/3880 is 17" x 37.5" using the Epson driver. If you use a RIP there should be no limit on the length. When printing longer than 37.5" you may run into issues with paper alignment resulting in skew errors. I have printed sheets that are 13" x 40" to print panos that are 12" x 37.5". They printed perfectly.
QImage is rumoured to allow longer prints, although using the Epson driver, by concatenating them. I've not tried it.
Even QImage can't do anything about the maximum width, though.