Epson's printer driver only support borderless printing on the following standard paper sizes:
4" x 6", 5" x 7", 8" x 10", 11" x 14", 16" x 20", and 17" x 22"
Although you can create custom page sizes up to 17" x 37.5", borderless printing is not supported.
I do not recommend tricking the printer into allowing borderless printing with unusual paper widths. When printing borderless there is overspray of ink off the edge of the paper. There are strategically placed sponges to catch this overspray. With unusual widths you a buildup of overspray on a part of the printer that was not designed for this.