Hi barelij,
Welcome to PC Mechanic !!
Experimenting is the best way to find out what meets your particular needs. The end use will also affect how you scan. For document storage, anywhere from 72 to 200 dpi is good, depending on how much detail you want to keep, or lose. Photo scan size also varies depening on what you want. 75 - 100 may be OK for email, but 300 is pretty minimum to print with, and 600 or higher is needed if you want to blow up the image.
HTH
TwoRails