A printer, when it lays a dot on the page, cannot vary the shading of that dot. An RGB monitor uses Red, Green, and Blue to mix togther and create all of the different shades of color. Just look really closely at a newspaper image to see what I mean, the dots have no grays in them, it's just black and white. This is why when you try to copy a black and white photo with a photocopier, it looks so bad. Because the copier can't copy the grays, it only has black and white dots to work with.
Some modern copiers have digital circuitry inside to create halftones from photos.
As you may have guessed, the image simply doubles in size. This is a given ratio. Scan at the resolution of your output device if you want it to be at the same size on the screen. Modern scanners such as the one on our classroom automatically can adjust for scale, so you only need to tell it the resoultion of the output device and how large you want it to end up, and it will do the resolution math for you. Remember not to scan at higher than you need.