In reply to cat88: If you are ordering online from some of the good outdoor shops, I would recommend calling them and asking them to try the crampons on a similar boot to yours. If they are a decent shop (I used Joe Browns and they were happy to do this) then you can trust that they know what a good fit is. Most will go small enough, but the heel issue is a pain. A good shop though will be happy to try them for you and tell you if they fit securely or not.
Obviously, if you have a good shop near you, you can just go in and take the boots and try them on yourself.