Quote - If you've only changed the preview size to change the render size, then yea, I can see how P8 would read that as your new preview size.

Yes, that's the problem. And I have 56 GB (compressed) of Poser scenes in this situation...
Obviously I'm not going to re-render them all, but still, it worked in P5, in P6, in P7, in PP, why shouldn't it work in P8?...

Quote - Can you undock the preview window?

I can, but that doesn't change anything. That's what makes it a genuine bug. I can understand the docked preview can't adapt, but the undocked one should, like it always did.

Quote - I guess a work around would be to go back into P7, and change the render dimentions to an exact resolution, even if it is exactly the preview size and save it again before you load it into P8?  (probably already been stated).

Indeed. But that's 5 minutes work for each scene... A lot of time to fix what should be working.
Oh well. I'll file a bug report.