I suspect that the larger the average paragraph size, the bigger advantage a bigger screen has for not losing track of where the paragraph is going, particularly a problem if it ends up being split over several screens.
On the other side of the argument, most current-model eReaders have fast enough refresh-rates, that it isn't
too distracting to glance back, if need be.
There quite simply is no hard-and-fast answer to this. The best thing is to try for yourself. The next best thing is to read some of the best-eReader-for-pdf threads on this (and other) forums. There have probably been dozens. The most recent one is
"Which eINK reader is the best at handling PDF files (ie, textbooks)?"
And
here are a whole heap more.