different frames, different sizes

I know you can’t animate render size in pixels but I’d like one frame to be rendered in horizontal view and one in vertical. I’m basically rendering stills but different views, so I animate just the camera setup and some other stuff (lighting mainly) and I need just different output proportions.

Create another scene from the first one. Choose Link Objects from the menu. The second scene can have a different render size and output. With a linked scene any changes you make in one or the other will appear in both scenes.