Hi Blackcherry. That's an 18-inch t-track plus from Incra. It's that length so that there's still room to move the piece on the x axis and still have room for the stops that designate the boundaries of the mortise. Any smaller (with this router fence) and it would be okay, but only to a degree. If too small there wouldn't be sufficient space for securing the work or stabilizing the work that lay on its side when being mortised. Any larger and there wouldn't be room to move on the x axis.