Yeah, you are going to need a theme component here and possibly to amend the avatar sizes to fit the new size + x2 of the size for retina.
You basically need to redo the widget to support this, its a rather hairy change, I totally support making this more easily customizable long term.
@eviltrout what the cleanest way of amending “settings” in the post-avatar widget, so size is something else? would reopen widget be able to pull this off?