First your scanning software should be able to set line resolution to 300 DPI, 24 bit color. If your image is larger or smaller than 5 X 8, take the height and figure the percentage. For an 11 inch original, 8/11 = 72% +-. for a smaller one, say, 4" height, 8/4 = 200%.
If you've got one of those funky scanners that doesn't have percentages or other tools Critical to Quick Work and just displays pixel widths of the scan area, set the "crop tool" in the scan software to the edges of the original. Figure the percentage like so:
5" X 300ppi = 1500 pixels 8" X 300ppi = 2400 pixels So you would increase resolution until the pixels read 1500 X 2400 for the scan area. This will give you a 5 X 8 at 300 dpi (DPI=dots per inch, PPI=pixels per inch, the two are synonymous.)
Once in Photoshop, select Image->Image Size. If the image shows a 72 DPI resolution, DE-SELECT the "Resample Image" box. Type in 300 in the resolution text field, and you should see the print size change to 5 X 8 when inches is selected.