@venno Hey, don’t worry. Welcome. Hmm, the problem is normally digital animation or painting software works by having a baseline resolution. If you change the resolution it will affect everything within a project. So, if you change the camera res to the higher number what will happen is that the rest of the animation will appear as if it was “smaller” than you originally intended.
I don’t know why you require such precise numbers, but no matter what video or software you work with, the final video will always be the size of the highest resolution you use, there’s no way to change the video size mid way. It’s either a square or a rectangle proportion.
With that said, if you kind of still want to do this you can try to export everything at 850 by 450 and create the illusion of size change by putting a black background that would fill the screen behind your animation as a base and on top, yet still behind your main drawings, put a square that acts as the “real” canvas that is the size of 500 x 450 and on the last frame you fill the background to be full white but at the final size (860 x 450). This would give the illusion of change, but the whole video would still be the same resolution.