Priniting out of LR: In Page Set-Up, select your paper size. In the Print module, for Layout Style select Single Image. go the Guides panel and make sure all of them are checked ON. Adjust the margins in the Layout panel so the Cell size comes out the way you want it for fitting the image to specific cell dimensions. The dimensions of the print you will get show in a little box on the upper left corner of the photo, so there are no surprises. Adjust the margins, Cell size and print image to what you need. Click Print. If you have Page Bleed clicked on, you can see how to adjust for what the printer requires so that the photo is centered and of the right size. See Martin Evening's LR CC book, Pages 454 and 455.