Quote - Do you have to do that before every export?

Yes!...
That's one of the regressions of Poser 8/Pro 2010. If you make a new scene and don't set the render size, it will be rendered in the default size of 500x490 or some such.
In Posers before 8 the preview size was automatically the render size, unless set otherwise;
In Poser 8, because of the docked window, the render size has to be set. Every time, for every scene.

But that will be fixed by the GUI, since it will allow to override easily whatever size Poser exports. Even better, since it will (AFAIK) allow to have setting presets, you can just load a preset.