It all depends on the resolution of the scan. If you take a print
the size of a negative and scan it at 2700ppi you should be able to
do the same enlargment that scanning a neg at that resolution would
permit.
the hitch is that most flatbed scanners don't scan at anything like
that resolution, in fact nearly uniformly they use interpolation to
achieve their advertised maximum.
So starting with the film and a very good quality scanner is the
better solution.
Now on the other end, for printing from the scan, the more info you
have from the scan the less junky the print will look, but there are
a whole bunch of facters which can "smooth" things over. For
instance, I have a middling 35mm film scanner. My printer is happy
to print any file that's 250 dpi at full print size and bigger than
250 ppi. However, when I use a glossy paper (like to make the pic
look like a photograph) things aren't as nice as when I use an more
absorbent paper (matte finish) which takes the little spots of ink
and sort of absorbs them a bit and smooths out the edges of things.
And when I take that same file to Wal*Mart and have it printed out as
an RC print you can really see the difference between the 250 and the
300 dpi.
Then there's what happens when you try to make the print 300 dpi at
much bigger than the original file. Photoshop has to invent pixels
and makes some obviously not very good decisions. There are a number
of choices then. One of them is highly disdained by pros and
probably cheerfully used by lots of amateurs. It's called Genuine
Fractals. It invents pixels better than Photoshop and the designer
of GF, who lives two miles from my house, will cheerfully assure you
that it invents pixels so well that noone can tell. But the pros
still insist they can tell. I've never used it.
But I can tell you, when I scan an image at film size x 720 ppi and
try to print it on glossy paper at 8x12 without interpolating, it's
ghastly. When I trot it down to WHOI and they scan it at 2700 ppi
and print it on their lovely big machine without interpolating, it's
beautiful.
So the moral of the story is get the best big scan you need for what
you want to print.
