I would avoid thinking about the final output size too much, except that (obviously) a higher scanning dpi will give you a larger image on your PC which will print better than a small one.
My thinking is that, who wants to scan photos over and over? Go in with the idea that you're "archiving" the photos and scan at a higher dpi than you need. If you're mostly scanning the same size prints (3x5 or 4x6 or whatever), try scanning the same one a few times and decide at what point you're not getting any more quality. This may be at 300 dpi, 600 dpi, 1200 dpi, whatever. Then scan at the highest dpi, and you'll have the best quality (that your scanner can do) when you do want to print it, or do anything else with it.
It's not a bad idea to keep a microfiber or similar soft cloth around and gently wipe the photos before scanning them, too - it's easy for hairs and lint and such junk to be on the print and you probably won't notice them until after they're scanned. Needless to say, keep the glass on the scanner clean, too.