I have mentioned this in another thread, but I guess I will need to try again to confirm it.
When using the parameters you're advising in the past, the input images that were 2k in resolution were being processed as 256x256(relative to parent like FAQ link states), the outputsize value given to sbsrender only seemed to affect the rendered output size, eg it could save images at 1024, 2048, 4096(with GPU engines), etc...however the content was blurry as if it had been upscaled the input image from a 256x256 size(despite external source being 2k).
The parameter name itself makes sense that it would affect the output image size, not adjust the default 256x256 size for sbsrender(where other supported programs have varying default sizes but from my understanding can be adjusted by those programs). For the time being, I manually set the resolution of the bitmap nodes to their source size, this doesn't render output images with that blurry upscaled look, instead they're crisp and detailed like expected.