The Kobo Aura One's details are leaked and it confirms a 7.8-inch e-reader. The Aura One is an e-reader with premium specifications - it comes with waterproofing, 512 MB RAM, 8GB internal storage and a 300 PPI E-Ink Carta front-lit display. Importantly, the listing states the price at 229 Euros - in comparison the six-inch Kindle Oasis is priced at 290 Euros. I don't think Amazon will respond with a larger e-reader. Amazon didn't respond when Kobo released the 6.8-inch Kobo Aura HD in 2013. For Amazon, size doesn't seem an issue and its near-complete dominance of the e-book market means it can set its own agenda. At the moment, it aims to gradually improve the range of six-inch e-readers, offering choice at that size, but there is no indication it is interested in offering a larger e-reader. However, I could be wrong.
On the other hand, it is the niche vendors that might need to revise their devices and pricing. Since the near uniformity of the six-inch e-reader, alternative vendors set-out to meet the demand for larger e-readers. Overall, the quality of these devices is sub-standard, with out-dated hardware and often poor software. Further, as larger vendors neglected the larger e-reader, the price of these devices are inflated. Eight-inch e-readers (e.g. Icarus Illumina XL, Pocketbook Inkpad 2 and Onyx Boox i86) retail at a similar price to the Kobo Aura One but with inferior hardware and software (PDF support is poor on Kobo e-readers but there is the option to install KOReader). Unless Amazon releases a larger e-reader or alternative vendors seriously re-consider their offerings then Kobo Aura One is the only serious option, at the moment, if Android is a non-issue for the end-user.