DPI and Image Resolution
Image resolution for printing is generally measured in dpi: Dots per Inch. The standard size for printing is 300 dpi. (That is, every square inch would be filled with 300 by 300 pixels.) At this size, the human eye cannot easily pick out the blurredness and imperfection of the image; from a normal viewing distance, it looks like a proper scene. Other outfits might recommend 220 dpi, which would be an absolute bare minimum. Anything lower than that would immediately appear blocky—also known as pixelated.
Images get pixilated when they are expanded past a certain threshold, where it is easily spotted with the naked eye. All images are composed of pixels, but when people are viewing an image, they don’t want to be overly reminded of this fact.
It is important to know what the viewing distance will be of the image. Images intended to be viewed further away—like a large poster, hung up on the side of a building, let’s say—can get away with more pixelation than an image intended to be seen up close. As a note about the MDL specifically: most people come in intending their posters to be placed immediately before the viewer, meaning the less pixelation, the better.
Various Resolutions and Their Sizes
Images come naturally in numerous pixel dimensions. All of them have their place in image work. Here are some examples, of rough image sizes and file sizes:
Size at 220 dpi
Size at 300 dpi
640 x 480
2.9” x 2.2”
2.1” x 1.6”
1024 x 768
4.7” x 3.5”
3.4” x 2.6”
1600 x 1200
7.3” x 5.5”
5.3” x 4”
2272 x 1704
10.3” x 7.5”
These dimensions are not hard-and-fast rules about image size; for a less formal poster, some pixelation could be acceptable. Stretching far beyond the dimensions listed under 200 dpi, though, will make the individual pixels of the image very noticeable, and thus it is cautioned against.
File size depends a lot on image compression and like factors which computers do automatically; determining any definite file size range is thus difficult.