I'm not entirely sure I read this correctly but your post seems to indicate
your 553 MB file is 6.3"x 21.1" at 1200 dpi. Is that right?

If it is, you only need a 300 dpi file for a print lab to return a high
quality print. The resize proceedure should be setting the dpi and size to
achieve the final file. 1200 dpi is unreadable by most printer's. If you
sent that dense a file to your home printer, the driver altered the size
before sending to the printer so it would have worked that way but for a
lab, only 300 or 350 dpi image.

As I said in a previous post, I've always been a bit confused by the
whole DPI thing, so I apologize if my post didn't make the best sense.
My scanner software gives me the option of setting "resolution" listed
in "DPI." I've scanned these 2.25" x 7" negatives at 1200 dpi, which
gives me a .tif file of approx. 70 MB. The lab tells me I have to set
the size I want them to print before I send the file, they tell me to
do this in Photoshopimage size. I want them to print the image at
6.3" x 21.1". When I set the image size to these measurements in
Photoshop, my file size jumps up to 553 MB and a pixels per inch of
about 390. According to one poster, this is a normal size for an
uncompressed pano. Hope this all makes sense now. I think I have it
figured out in any case.