uBlock does not know the size until the file is requested. At that point it is too late, you have accessed the tracking file. That feature in uBlock is meant for people trying to save bandwidth/CPU. uBlock is great though, I would never browse the web without it.
Edit: I forgot the grandparent mentioned them. Ublock Origin implements the parent's proposal,so I mentioned it.
Focussing on file size is a mistake because you can just make the tracking image display something decorative or whatever. I was referring to a solution for not loading third party resources without manual approval, which would need to be more general than just blocking single pixels to be robust (although hueristics might be good enough for some cases)
