Printing acts the opposite of video in almost every way.
Image program and printer driver options can cause results to vary. The software may have various options to make the printed size be different, to fill the page for example, or to scale the image. It is difficult to specify exactly what will happen in every case, but if it isn't going well, look for some obscure option setting affecting print size.
In most programs like Photoshop or PhotoImpact or PhotoDeluxe, the scans are printed at real size. That is, if you scan a 6x4 inch photo, and then select the menu FILE - PRINT, it will print 6x4 inches on the paper too. Resolution does NOT determine image size on the printer as it does with video. The size of the original scan area determines printer image size. Lower resolution can look fuzzy, and higher resolution may look better, but the printed size will be the same at any scan resolution (scaled resolution is something quite different, discussed below). Printed resolution works like we think of resolution, as increasing image detail instead of image size. But even so, like video, there are limits; we are still very much limited by the capabilities of our output device.
Printed pages are normally a standard size defined in inches, 8.5 x 11 say, so printed image size in inches is very meaningful too. The printer driver will try to print the image at its original size in inches, unless told otherwise. Conversely, video screens don't care about inches. If you have a 400x400 image, video monitors will show it as a 400x400 image (unless the viewing software's goal in life is to make it fit or fill the screen, etc). Notice that both techniques do maintain the same relative size of the image to the total size of the page or screen.
To print images at the actual original size on the paper, the value for resolution entered into the scanner's TWAIN driver is both the scanning resolution and the printing resolution (assuming 100% Scale Factor). The same resolution (same dots per inch) is what makes the image print at actual real size. Scan and print, that's about all there is to it, unless your printing software interferes with its own notions about how it should be done. TIF and JPG image files store this resolution number in the file for later use (GIF files do not), and good applications will honor it. Some applications do not.
But when we want to print the scanned image at a different size on the paper, there are special techniques and concerns for scaling the image size. Since this is the norm instead of the exception, we'll give it special attention here. Scaling is a word that means stretching or shrinking the image to fit a specified area, and it is accomplished by simply changing the value of the number used as resolution when the printer calculates the spacing of the dots on the paper. Nothing else happens, except that the size of the printed image changes. It's a simple operation, but the concept is a little abstract. It's much harder to explain than to do. I'll spend some time on it if you're interested in the whys of it. Don't let all my words scare you away, that's just my own shortcoming, and scaling is really quite easy.
Scaling is an extremely important working tool for printing. There are two ways to scale an image, either during the scan, or after the scan. The next page covers scaling AFTER the scan.