Well, despite not having pixels as such a film print still has grains of ink that form the image. From a certain resolution they would become apparent instead of the image elements and probably wouldn't help the enlarging process.
So the aim is to get as many usable pixels as possible.
According to this article the "resolution" of a 35mm film is about 4000 DPI. I see your scanner should be capable of about twice that. So if you go beyond 400 you would probably get visible grain particles. Sounds to me like you would need to scan close to the given 4000, see if there's visible grain and if there is go lower and lower until you get a relatively clean image (as it may depend on the film quite a bit).
Edit: I sort-of overlooked "prints" and talked mostly about film scanning, but the same rules apply, just, way lower DPI, like Artisan-West said below. In theory 300 dpi is assumed to be the limit of human vision, but if the print is of high quality it might have more (not likely I guess). But the approach would be the same - start high and go ever lower until you no longer see a pattern of dots.