Leslie,
Be aware that you will want to scan a fairly large image, or have that image be very very crisp. Regardless, I would advise that they scan your pics at at least 600dpi.
An example: You have a 4x5 pic scanned at 300dpi. It will have print quality at 3x5 inches, but if you want to enlarge it to say 8x10 inches, then the resulting print will have a resolution of approximately 150dpi - pixels start appearing. Scan the same size pic at 600 and enlarge to 8x10 and you are still at 300dpi. If you get an 8x10 done and then scan it at 600 dpi, then you have flexibility up to almost double that size.
Something to think about...
