The act of enlarging the photo will degrade the quality automatically -- not of the originals, of course, but of the digital copies. The trick is to scan the photos in at a high enough level of quality that the human eye cannot tell. I find between 300dpi and 600dpi sufficient for this, depending on the image. The process is relatively straightforward, but the images you're scanning will dictate part of your process. You should be able to find many different tutorials on YouTube regarding how to enlarge and enhance photos using Photoshop.