An image _has_ no size. A piece of paper does, the scanner does,
your monitor does, heck the size of your living room does. But an
image doesn't.
Imagine taking a tight close up pic of your grandkids face. How big
is it? The size of the sensor? The size of your grandkid's face?
It has no size until we do something physical with it. Print it 4 x 6.
the the piece of paper will be 4 x6. Same for 30 x 16. Projected on
an Imax screen it will be dozens of feet.
So, in the OP's case all he needs do is provide sufficient dpi to
allow "me" to print it at whatever size I might like, or he may
recommend. Then it's up to "me" to print it as I like.
Hi Ken. Theoretical debate accepted. Yes, digital images always have a
size, but the dimensions of that size is in pixels, not in inches. For
example, a 1024x768 pixel image. Paper is dimensioned in inches. But video
screens are dimensioned in pixels, that 1024x768 pixels again. The image
size in pixels is the most fundamental aspect of its inherent properties.
There are always a few IFs and BUTs possible, and even though the image is
dimensioned only in pixels, image files generally do contain a dpi value,
often the scanning resolution by default (assuming 100% scale). This dpi
value is just a reference number stored in the file for future printing
purposes. The dpi value predicts a future image printed size in inches on
paper (dpi is pixels per inch, the inches are on paper, and the pixel
dimensions will cover so many inches of paper at this pixels per inch
value). In that way, the image file does define a future dimension in
inches on paper, when it reaches paper.
Our photo editor's resize dialog will show all of these numbers, the image
size in pixels, the dpi printing resolution value, and the future dimension
in inches on paper at that dpi value. Except yes, we can rescale to change
that dpi value at will of course, and thus change the future printed size in
inches, up until the time we actually print the image on paper and it
becomes history. Only the size of the image in pixels is actually an
inherent property of the image.
But if we dont arbitrarily rescale it (dont change the dpi value), the image
file will contain the scanning resolution value (assuming 100% scale), which
will be stored in the file as the printing resolution value. Thererfore,
(with scanning resolution equal to printing resolution), the pixels and dpi
and inches always work out (assuming 100% scale) so that the image will
always print at original size again (unless you rescale it to another size).
This is true for any scanning resolution, which affects quality, but not
printed size, as such.
We can easily scale any image to any printed size, but I cant imagine any
way that simply changing scan resolution (before the scan) will affect the
printed image size on paper. Such a resolution change would of course
greatly affect the image size in pixels, and therefore does greatly change
the image size seen on the video screen, but scanning resolution alone wont
affect the size printed on paper. Some other action is required.
So I assumed the OP needed to know that scanning at the 100% scale setting
would always print at original size, regardless of resolution. I have not
figured out what John might be seeing, unless he is judging size on the
video screen. But the video screen is about pixels instead of inches.
To judge printed size, check the image resize dialog where future printed
size on paper is shown. Or actually print it.