How much size upscale or size downscale before pixelation or degradation of image?

[Blighty]

(01-17-2019, 01:14 AM)Tetragram Wrote: But the heart of the question is, how much upscale and downscale generally can be done to an image before it degrades or pixelates in Gimp?

Any upscaling or downscaling in any bitmap editor will degrade the image.

The real question is: When will the degrading become visible and unacceptable?
That will depend on the image itself. And it will also depend on what the image will be used for (eg viewing distance).