You think IR's resolution is percentage based, but it just uses the current window size. If you want 960*540, click render with that resolution, stop the render. Now your window is adjusted to that res and IR will render 960*540. This behavior is introduced to fully integrate into Quad Panels.
Maru meant, that it does not matter how big the frame size is,it can be random, as the purpose is preview. I understand your position, where this is not desired, as it shifts your understanding of the frame. You want the preview to be the same as the final frame size or same aspect at least. This is however not how IR is intended to be used and it conflicts with your preferences
I hoped I guessed right :D