I think a 12 inch screen would make for a bulky and heavy reader which few people would buy. So I doubt Amazon will make one.
Don't forget the you can rotate the DX to landscape which makes it easier to read scientific papers. The important thing with PDFs is to use a program to crop out the excess margins.
For old papers and books that are scanned and converted to a PDF, few of them will look good on any device, not even a computer. There needs to be a mindset change among those who publish scientific and technical journals and books. They need to understand that large sized docs that cannot reflow their text are not good for digital viewing, even on a computer. They need to either publish in a reflowable format or at the very least design their docs for smaller screened devices. It is sad that the worlds greatest minds seem to be the last to understand this whole digital publishing concept.