Sorry, i don't plan this feature.
This would be a great option to have. Affinity Photo for the iPad has this and is very helpful for saving time on export with size and format.
On export Im having to resize first then export and make sure I undo the resize before the autosave. I've made this mistake and my project has been saved permanently downscaled. And if I was to do as you suggested by duplicating the layer and making a new document I can't just export from there and close the file without having to go back to the home page and then delete the newly saved version of it.
If a sizing option was on the quick export and main export it would certainly be hassle-free and make it flexible.
Also is it possible to have the option to save or not save (while autosave is on) a new image without it automatically saving to the home page? Especially if you drag an image into your work window as a new document, not as a layer.