Hello!
Old thread, but the issue is the same. I pretty much never used it. It was always a nuisance. When I open paint.net not directly with an image, I have an image on the clipboard, which is useless to paste on the pre-determined sized canvas. I always need to close it and then open it again. Otherwise I may want to drag&drop an image, in which case the default canvas is also of no use.
I think it would be more productive if paint.net would show a few basic options, without creating a canvas. Such as, create canvas from clipboard image (dimmed if no image on clipboard), open image, create new canvas, maybe recent images option. Allowing people to chose what they want to do instead of presuming they want an empty canvas, with the size pre-set.