Alright so, you know how sometimes when you go to make a background larger than the actual project itself, but when you go to resize it with actual Size blocks, scratch will try to autocorrect it to make it fit within the perimeters of the project? Well I may have found a workaround that might be of use. It's really as simple as creating a blank costume, and using the block setup I've shown in the project Name the Blank Sprite anything you want, for the example I've just called it "SizeBruteForce" for practicality, and make the size whatever your heart desires, that's basically all there is too it though. Now go out there, make those overcomplicated backgrounds using sprites and (hopefully) never get your hopes and dreams crushed by Scratch's janky engine ever again.
