Computers don’t work in inches, only virtual squares called pixels. You know how if you look at a newspaper under a magnifying glass, you can see the little dots that make up the pictures? It’s a similar idea.
So the thing is, you can choose how many pixels you want per square inch. The higher the count, the sharper-looking the image, but also the bigger the filesize. If you printed something with a too-low pixel resolution, it would come out looking something like this.
In the print industry, 300 dpi (dots per inch, roughly equivalent to pixels per inch) is typical for a good-enough looking print. At 8.5x11 inches, that means you need 2550 x 3300 pixels, or 8,415,000 total pixels (8.4 megapixels).
What are you trying to do with Paint? If you’re trying to blow up an existing image into 8.5x11 size, it’s unlikely to produce good results.