Brandon's Way of Kings is 1500+ pages at the moment (just for comparison's sake), so yeah, Martin's new one will be large.
As for anger/frustration at Martin? I think it is completely justified to an extent. We are reviewing an anthology he edited and wrote for, and it bugged both Nick and I. I personally feel that he lost the drive to finish the series. Now, saying you are frustrated is one thing (as I am). I do think it is important to be somewhat careful of how you express those frustrations though.
And comparing the next book in a series of ASoIaF to the time between the Hobbit and LotR is like apples and oranges. The Hobbit is an entirely different animal from LotR. A lot of people almost treat them like they were written by different authors. The tone, complexity, pacing, and everything is different. This isn't the case with Martin's stuff, and we are in a completely different era of writing.
But yeah, my personal opinion is he lost momentum, and decided he'd rather work on 87 bajillion side-projects. And saying he doesn't owe the fans anything is false, and borderline Goodkind-ish.