Typically you scan at the highest optical resolution of the scanner and save that as your master file. Then you generate down-sized samples for individual uses. Even if you want smaller files for storage, it works better to oversample and downsize.
Depending on the scanner, a quick internet search can give you the real world low down on what the resolution really is. Usually the marketing copy for the scanner lists its interpolated resolution that's often double what it really can do. I forgot what they advertise my Epson as being capable of but the reality is less. But it's still sufficient for a large ~ several hundred MB file from a 6x9 neg.
