Some time ago, I purchased a software/hardware integrated device that is capable of creating huge images. It can be tricky to use, and is always cumbersome and takes a fair amount of time. However it’s possible to make very large files.
People typically talk about billboards when discussing really large files. But I think billboards actually don’t need such high resolution since they are typically viewed from a distance. These images would be suitable for covering walls in stores or offices and the like. Situations where viewers could see the whole image from a great distance, yet walk right up and see a nice clear picture.
Some photographers simply up interpolate a smaller image, meaning they just add pixels- and that won’t help much in an extreme enlargement. For a huge print to look good close up, the minimum native resolution should be 100, maybe even 72 ppi. That’s about the same as your typical computer monitor. That wouldn’t be appropriate for “fine art” type prints, but would be more than acceptable for a ca. 8×20′ (2.5×6 meter) wall.
Creating such large photos and prepping them takes a lot of time, but I’m gradually adding them here. At present the finals I’ve created are layered tif files, and through a quirk with Photoshelter they display strangely. I’ll have to sort that out, but there’s also a screen capture with the resolution open in Photoshop.