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From what I know about printing, a smaller file will not turn out as good as a larger file will. That's using a jpeg file. Other formats such as RAW and TIFF will probably turn out better printed matter over jpeg but are very large in file (in mb) size. It's as aoie says, jpeg is a compression format and gets rid of certain information to get the smaller file size. So if you want to print a jpeg file it'll need to be very large file size as in 4000 X 6000 as opposed to 800 x 600. Also, the compression ration of the jpeg file will play a role too. Professional printing places might be able to do more with a file than the average Joe can. And they might have stipulations about the picture you want printed.

Here's a link http://www.scantips.com/basics09.html
And Wikipedia's take http://en.wikipedia.org/wiki/Graphics_file_format
And you can check this out http://www.allgraphicdesign.com/scanning.html