To make a 16x20 from a 6x7 negative, you need to enlarge from 2.5 to 20: a factor of roughly 8. That assumes no cropping.
To get an 8X enlargement and print at 300 dpi, you need at least 2400 spi. While some have gotten close to that figure, I can't help noticing that the contrast at the high end of the resolution scale, is quite low. Some might be able to distinguish bars on a USAF target, but I'm more comfortable with my 700, when considering an upper limit of 1600 spi. I don't want to spend the time scanning at 6400 spi to squeeze the extra data out, if there is any.
1600 spi allows a 5x enlargement: enough for an 11x14, but not beyond.
If you shoot clouds or other soft subjects, then you might be able to get better results, but if you're shooting brick walls or other demanding subjects, you'll be WAY happier with a dedicated film scanner. They are scarce, but that's another matter.
I got a 700 after using a 4990, and have not seen a dramatic improvement.
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