Most everyone scans at too high of an image resolution. This captures so much detail that files become too large to work with, slowing or crashing your computer.Generally, a good rule of thumb is to scan at 2 to 2.5 times the line screen you will print at (for an image at 100%).
We usually print at 133 line screen, so if you scan a picture and don't stretch or shrink it in your layout program, 300 ppi (pixels per inch) is a sufficient scan.
Scaling the picture adds in another factor. If you stretch your picture from a 3 inch square to a 6 inch square (doubling its size), you reduce your image resolution by half. Three inches of information gets stretched out over six inches. Likewise, if you shrink a 6 inch scan down to 3 inches, you double the resolution. Six inches of data is compressed into three inches.
So, either try to scan at exactly the size you'll use your image or remember to increase or decrease your scanning resolution as needed. If you want a 6 inch photo to eventually print at 3 inches and 300 ppi, scan at 150 ppi because your resolution will increase when you scale the photograph down.