If I recall correctly it's 0.3 inches on each side.
The Epson drive (on my Mac) has a wide range of paper sizes for bordered and borderless printing. 17x25 is not among them, so you'll need to create a Custom paper size. That option requires the default borders.
I print some 17x25 sheets, printing 16x24 inch images with 0.5 inch borders. I like the look. I use the house brand luster paper from inkjetart.com.