SilverFast offers a preset and a resolution slider to control the input and output resolution of your images. If you plan to blow up an image, you need a resolution large enough to allow for the necessary scaling of the original image. You can set this in the scan dimensions dialog:
If you know the quality of the printer that will be used and the size of the paper that will be printed on, SilverFast can calculate automatically which input resolution will be necessary for that.
If you don't know yet, which size you'll need your images to be, using the maximum optical resolution is a good idea. It is the best resolution that your scanner can achieve physically without interpolation. The file size is on the large side, but you don't have to rescan if you need a large image later.