Here's a help document we wrote to help out folks with this:
But to help you more, I need to know if you're actually printing this out, or you want the pixels to show up 8.5in across your screen?
Pixels can print out at different sizes when sent to a printer. This is controlled by the dpi setting in an image. If you have more pixels, and you want them to be dense, then you'd set a higher dpi. You can take the exact same image, and change it's dpi from 72 to 144, and it'll now print out half the size a previously (but the image will be sharper, because the pixels are packed together tighter!).