First, if you are running canvas and not choosing a canvas setting on the printer try to switch to a canvas media type and print again.  The different instructions for feed are built into the media types that Epson has on their printers so this simple change could possibly correct for your length discrepancy.

If that does not help there is a slider inside of the Epson Driver. The "Paper Config.." button is located on the main tab of the driver next to the print quality dropdown.  Clicking this tab you will see a slider for "Paper Feed Adjustment".  If you see solid bands of color in your print (feed is overlapping) and/or your print is coming out short you need to adjust to the positive side.  If you see white bands and/or your print is coming out too long then you will need to adjust to the negative side.   To account for the amount needed to adjust you will need to trial and error print with the exact material you are trying to dial in.   Start by going half way to + or minus 35 and work your way back or forth in halves until you reach the proper length.  NOTE:  Keep this number handy and any other media setting adjustments made for future use of the product.  You can also save your adjustments as a preset with the Save/Del button next to the select setting drop down at the very top of the printing preferences window.

Slider (Highlighted) is located here: