Not all cameras have "pixels". Back in the good old days, we had something called "film". Many professionals still use it. Not unlike pixelated images, though, even film will lose its focus if enlarged too big.
Digital photos can also be put into a vector form. Have you ever heard of SVG files? Instead of color-for-color, pixel-by-pixel images, an SVG and other vectored imagery is based on a single center point, direction and relative distance... they are scalable without loss in resolution.
Otherwise, all photos can be enlarged to any size. And film itself comes in different sizes, as well as the MP's of a digital.
I know it doesnt fully answer your question, but it might be a start. Hope it helps. Im a novice myself when it comes to real photography.