Our barcode measures 1.25" x .5" and already includes a white box.
After you upload your artwork and click Save and Continue, you'll lay the UPC template (provided in the ART section of your project) over top of your completed image.
You should account for this space, but it is far easier to simply leave this space empty than to fill it with white (or a color), as attempting to line up one white box on top of another on our site can lead to frustration or the appearance of a misprint on your final product.