large prints
if you do opt to scan them, I agree with the first poster: a dedicated film scanner will yield better results than a flatbed.
In any case, what you want to do is figure out the resolution you will need to scan the image(s) at to yield your size. A resolution of 240-300dpi is best; however, once you get into oversize posters and/or billboard sizes, because they are being viewed at longer distances, the dpi can be as low as 100 to 150 in most cases. If you are going to have them printed professionally, and not do them yourself, the printer should be able to tell you the resolution that their plotter prints at.
Anyways, an example would be that you want to print a 18" x 24" size of an image. To get an acceptable printout at that size, you would need a minimum of 4,320dpi x 5,760dpi (at 240dpi) or 5,400dpi x 7,200dpi (at 300dpi). Also, you need to make sure that the scanner can handle those resolutions natively and not through interpolation, which just adds pixels.