If you set it larger than 72, the image will be clear but large. If it's smaller, the final image will be smaller. In some programs, including Ofoto, there are two resolutions to set. The first one decides how much information is read from the picture and the second one decides how much information is saved. The first one should never be smaller than the second.
Most programs have preset choices.
If you choose one of these, it should be color or black and white photo. Next, you can usually choose to do a prescan. This will let you make sure that the picture was put in the scanner the right way, etc. You can repeat the prescan until you think the picture is straight. After the prescan, you can select the area that you want to scan and save.