filmscanners: Film Scanner Question Again

[Rick Decker <rickdecker@hawaii.rr.com>]

I have 3 parameters on my 1640SU scanner - "Source Size" , "Target Size"
and "DPI".  The manual tells me to "Increase Resolution" as I increase
Target Size.  This can result in some horrendously large files and I
suspect there is a limit above which the file gets bigger but does not
contain any more data.  The manual talks about 300 dpi resolution and
higher as the target sizes increase.  The question I have is if my
target is 8x12 for a 35mm slide what should my resolution be (DPI).
Here is what the reference manual says "You must manually adjust the
resolution (DPI) to maintain the same image quality when you resize your
image.  For example, if you have a 2x2 300 dpi image that you want to
enlarge to 4x4 (a factor of 200%), increase the Scale setting to 200%
and change the resolution to 600 dpi".

Now let's say that I want to take a 35mm slide (1x1.5in) and enlarge it
to 8x12...my resolution would be 2400 according to the manua
(12/1.5=8...8/1=8...300x8=2400)l.  At 1600, I would have a file size of
705 megabytes!! I am sure that this is way beyond the point at which the
increase in file size does not result in any more increase in data.

It sounds to me like I should leave my resolution at 300 dpi/ppi...even
that will give me a file size of 24megabytes which I suspect is larger
than I need.  For an 11x16 it would be 45 megabytes.

Any advice is much appreciated.