Hey Roberto:
You are ahead of most folks in that you know the difference. I think most older 300 DPI inkjets use 7 or 8 dots per pixel. You could just be conservative and multiply the scanner DPI by 6. So if you scan at 200 DPI that is at least 1200 DPI of printer resolution. Some people use pixels per inch (PPI) instead of DPI to differentiate the two.
Most newer high resolution printers use much smaller dots and have at least 11 dots per pixel. On the HP printer forum there is always someone complaining that he or she scanned their image at some extreme resolution and Photoshop comes up with a little box when you print saying it is sending the image to the printer at 300 DPI. So they have a 1200 X 2400 DPI printer and Photoshop will only send a measly 300 DPI. Actually, with at least 11 dots per pixel the PRINTER resolution it is sending is over 3000 DPI and the print spooler is going to have to downsample the file to print it.
http://scantips.com/basics03.html
There is no more than 200 scanner DPI or PPI on a good photograph. If you follow the links to his test example, Wayne Fulton uses a photo he took with a fixed Nikon 50mm lens on a tripod to demonstrate the point. He also provides some good references:
http://scantips.com/basics08.html
If I am going to print on photo paper at 1200 X 2400 I scan at 300 DPI even though it will not improve detail over 200. I do that for several reasons. The printer instructions suggest 300 DPI scans for printing at best quality on photo paper so it can downsample the image rather than upsample it. If I want to blow the image up in Photoshop to enhance it I have more pixels to work with even though it is no sharper. I also have 512 Mg RAM so I can be a little cavalier about image size.
You can’t go wrong scanning at 200 DPI for your printer. You get all the detail available on the photo and your printer driver will definitely have to downsample as there will be more pixels than it can use.
If you want to print the photo larger than the print you scanned there are a couple of ways to do that. Be aware that quality will suffer a little as there is not enough information on a print to make a really good quality blow up no matter what equipment you have.
The first way is to set the size you want in the scanner software. Most image editing software will print at the size the scanner sends it if you don’t change anything. Be aware that if you scan a 4 X 5 print with the size set to 8 X 10 and the resolution at 200 DPI, the scanner will actually scan the photo at 400 DPI which is excessive. I would set the scanner for no more than 150 DPI in this case giving you a 300 DPI scan.
The second way is to scan the photo at regular size and have the image editing software resize it to the print size you want. In Photoshop you have a choice between resampling to a desired print size or to a size in pixels. You want the print size.
If your editing software has the option, you could also set it to “fit to page” as Sandgrouper suggests. This gives you the largest print size your paper will allow.
http://www.scantips.com/basics02.html