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Question: scanning slides

I'm scanning photos, prints, transparencies, and slides of original art, all at very high resolution (6400 & 9600 dpi) so that the finished images can be as large as, say, 10' wide at 300 dpi. Problem with slides: is there a way to overcome the tiny dimensions in the scanning process? Epson won't let me change the initial scan size (message that change is "out of range"). So, even scanning at 9600 dpi, the images are only 2-4' wide at 300 dpi. Any advice?