Scanning and saving

I scanned an envelope and another object covering a regular 8.5x11" surface as a photo and then saved it as a jpg file. The jpg file, however, is very large (2549x3509 pixels). I would like it to appear on my monitor on its actual size.  Of course I can resize the image, but than I have to calculate how many pixels correspond to an 8.5x11" area. Is there any way to have the image saved by the Image toolbox in the size that I want?