Is there a way to force pixellate the viewport render? I occasionally use blender for pixelart by disabling interpolation, mipmaps and anti-aliasing in render options, but it would be great to see how a model is looking without having to render it everytime.

The render preview in the viewport only comes in one resolution, and it would be great to view it in the exact dimensions or just overall look as the final render. is such a thing possible?

Here's a render with the look I'm going for side by side with the viewport view in render mode.
You can change your viewport resolution.

Render Tab > Performance > Viewport > Pixel Size