I don't understand kyuss why you say you can't print a given file at a large physical size? You can always divide your pixels into smaller pixels, so you can make the file as many dpi as you want (even 12 million dpi). They will be highly correlated and probably look cr*p when printed out, but there is no technical reason you can't do it. The resulting image should only be limited by how clever your interpolation algorithm is.
Of course there's no technical reason you can't. I could resample an image big enough to cover the face of the earth, but like you say, it will look crap. You're not really dividing pixels as such you're adding them where none exist. The interpolation programme has to make a guess at what these 'missing' pixels should look like so while you gain physical size you actually lose sharpness and detail.
Here's a very simplistic example below, shown at actual pixels.
The original at 5cm wide 72dpi
If I make that 300dpi for printing, to keep the same amount of pixels as the original image and therefore keep the quality, its physical size then has to shrink and looks like this
If I was to enlarge the original, by either increasing the dpi without reducing the size, or by increasing the size without reducing the dpi, or even both (essentially adding pixels to the image) it looks like this
No one could argue that the big image looks as good as the original. These were just done in Potatochop and there are much better interpolation programmes out there. You could also add some sharpening and a few other filters to get a reasonable image, but there is a limit.