Digital. You can do it optically by setting your enlarger horizontally and taping paper to the wall, but it is a huge hassle, even with a big internegative. I've never done it and don't want to. Go for a good digital scan. Don't use Velvia, it will be too garish for people. Use a more neutral film, like a Portra 160N, Astia, etc. You can get away with a faster film, because people will move back to view something that large.