You can't put anything on top of the canvas. The reason it doesn't allow for that is because whatever you put there would sit on top of users avatars when viewing on mobile devices. It just doesn't work responsive wise. You can only place fields down within the box where a users name is. There are 2 positions there. Canvas Stats and Canvas Info. Their placement is as follows.
Regarding image sizes they only take affect during upload. You cannot upload, change sizes, and expect it to reprocess every image ever uploaded to that field it does not do that and is unlikely to ever do that as it'd just crash servers trying to process that many images.