lalebard, I got a 9.2" Pocketbook (similar to Onix M92, but no pearl screen) for the same reasons as you, but one thing I had not considered is that larger screens are more fragile, you definitely have to hold the device with two hands (surely pick it up with two hands) to minimise the risk of breaking the glass substrate, and the short of it is that it is not really portable, so on work trips I ended up reading academic pdfs on my 5" device in landscape mode. I've done it for years - not ideal, of course, but it can be done, and I've read academic techincal books (statistics, maths) on it for hours. What I found of paramount importance is that the pdf renderer is good. Pocketbook's is excellent, but now you have koreader on Kindles and Kobos, so they become an alternative, too.
Personally if I did not still have the Pocketbook, I'd go for the Aura HD, which I am told has a great definition (but I have never seen it in the flesh) and a relatively large screen while being only marginally larger than the PW (which is larger and heavier than the Aura non HD).
I think it is possible to read on 6" devices (in which case the
Pocketbook Touch Lux would also be an option, their pdf viewer is really very good, and you can also read djvu files) comfortably (at least I can), but this is a very personal thing, and as you have seen others here do find it an absolute no-no.
If you would have no other uses for your ereader if it were not to perform as you want it to with academic pdf, I'd try and see if you can borrow one from a friend and see how you find it, though I realise this may be quite hard to do.