Ok, I might not say this properly. Hopefully you will understand what I meant rather than what I said. Anyway...
Wouldn't something like what you're referring to be device screen size dependent? I don't Save to Device/Disk, so I'm commenting without any real experience. But as I understand the conversion process, one factor that calibre takes into account is the screen size, adjusting output accordingly.
So size is relative. Large on a 4" screen is one thing, on a Kindle 3 its something completely different. Unless it could be checked via a percentage of screen size: get image_size. if image_size >=75% report possible_cover:=true.
or something roughly equivalent to that. Margins and automatic image resizing might interfere with that idea however.
PS: I don't exactly care for double covers, particularly: two copies of the same image; or, a generic (calibre?) cover and the real publisher's cover. I might not have a problem with multiple cover images from various editions of a book - provided its a smallish number of images total (<12?). Forget something like LotR/Hobbit with hundreds of editions worldwide.