I wrote an article on all the ways I know to get extra size out of a photo. I’ve already mentioned it in the blog, but recent questions show that I need to spread the word again. Blow Up 3 gives sharper results with less computer artifacts than previous versions, so you could stretch the advice of that part of the article a bit.
Below is an extreme enlargement from Sandra Selles-Rodriguez. You can see the original beneath the big print. That is way beyond what we normally recommend, but it shows that you need to just try a sample and see how it looks on the wall. Some photos look much better than others in huge enlargements.
Below is a six part landscape from Sandra’s front page that was made with Blow Up and Snap Art. Snap Art helps you go larger because the original pixels are replaced with brush strokes that provide their own detail and sharp edges. For the best quality, enlarge first with Blow Up and then apply Snap Art.