Oh my God, my (annoying) experience yesterday should help you out.
I print my covers at Staples, yesterday was first time printing UGC ones (8.5x14). Tell them you want the 8.5x14, unfortunately at least my location only has the basic quality of paper (20lb?). When you bring your images on a flash drive, be sure they are all saved in .pdf format.
I brought my images in .jpg format, and she had to convert them to .pdf. After asking 3 times if that would resize them (and her confirming that it would not), I got them printed out. Got home and found them WAY too big for UGC covers. Went back and they reprinted them using my saved PDFs. It will save you a lot of time and hassle to bring PDFs and tell them to just print on 8.5x14, keep the original sizes.