You want to print using LR's print module right? This print size is based on resolution of the image itself and the Cell Size you have in the Print module. First thing to do is go there and make sure the Dimensions check box is on in Guides. Then you'll see that as you alter the margins of a cell, the resolution of the image updates as does the size of that photo when printed. Of course, you need to pick a paper size first (so I'm assuming you want something smaller than 4x5). You may need to create a user template of the paper size; what is that?