If you ever need to print a pixel art illustration, you'll have to address a few issues for the graphic to print the best way it can.
Pixel art is possibly the least resize-friendly illustration style, so let's figure out what we have to do to adapt it for printing.
1. If Size on the Page Is Irrelevant
Perhaps you want to print the graphic just to see it on paper, without any other content on the page and no layout concerns. That being the case, the process is pretty straightforward.
Create a new document in Photoshop, set to letter size or whichever preset size works best for you to print. Leave at 300 DPI as that's pretty standard whether you print at home or at a shop.
Then paste your pixel art piece into this new document. It'll probably look very small on the page.
300 DPI is a high resolution, and a letter sized page at that resolution equals 2550 x 3300 px which is a lot of canvas for pixel art. So let's resize the graphic.
Let's say we want to print it as large as possible. As my graphic is longer horizontally, it would make the most of the page if I rotated it.
And then to resize it you must make sure to increase the height and width, maintaining the aspect ratio and in multiples of 100% (so: 200%, 300%, 400%, etc.) otherwise some pixels will be bigger than others. And you must set Interpolation to Nearest Neighbor, otherwise the pixels will get blurry.
As I rescaled to 500%, each pixel on the illustration becomes 5 px wide by 5 px tall.
The result at 100% zoom level shows us large, evenly sized and perfectly sharp pixels.
You can now print this out!
2. If Size on the Page Is Defined
It may be the case, especially with commercial work, that a size for the illustration has already been determined.
So let's now learn how to adapt or create an illustration with a specific size in mind.
We'll start by creating a new file with the size we'll use, which will be 7.5 inches wide by 5 inches tall (or whatever you might require) at 300 DPI.
We'll shrink this to a working canvas size and then blow it back up, enlarging the pixels.
How much we shrink it depends on how much we'll later enlarge it. Enlarging to 5x works pretty ideally for magazines and other documents that'll be held at arms length or closer. For a poster or something to be viewed at a greater distance, on the other hand, you'll want to have pixels larger than 500%. Just remember to keep this rescaling to multiples of 100%.
We'll go with 5x (or 500%)… so the working canvas size will be 1/5 of the print size (or 20%).
You can fill a new layer with some random color and rescale to 20%.
Now you can copy this smaller rectangle and create a new document; Photoshop will automatically assign it the dimensions of the rectangle in the clipboard.
This will be the new working size. If it feels too small and restrictive, you can try a step higher in resolution: shrinking the original canvas to 1/4 (or 25%) and then enlarging 4x (or 400%) which would mean smaller but still visible pixels at about arm's length. Higher resolutions would still be possible but you might start defeating the purpose of using pixel art, if the pixels are not discernible when viewing the art.
Now you can work on this new file to your heart's content.
Additionally, you may want to expand the canvas just to more freely work on the edges of the illustration, but preferably keeping a reference of the bounds of the visible area.
Now we copy the art back to the large resolution file and enlarge to 500% to fill the original canvas, maintaining aspect ratio and with Interpolation set to Nearest Neighbor.
And you're done!
Ready to Print!
So now you have a file that at the 300 DPI resolution fills 7.5" x 5". This is the space it would use in a letter sized page:
And here's how the art looks at 100% zoom:
With perfectly sharp pixels that would mean clean, ideal printing, as long as it doesn't get any more resizing, of course.