They've never said that would be the case though, have they? I'm sure the finale and maybe 1 or 2 other episodes will hover around 90 minutes, but I don't think anyone should expect six 2-hour episodes.
It seems more like the episodes are just harder to make, and not necessarily going to be longer because of it. Like this White Walker battle sounds exponentially more difficult to put together than Battle of the Bastards, but I'm not expecting it to be two hours long because of it. It'll just be a really involved and packed 60-70 minutes.
They have said some of them will get that long yes. Not 2hrs long, but up to 80-90 mins for multiple eps I presume.