I have to say, I am a huge shoe lover But I don't have a pair of iron fist. They really gorgeous though.
My friend bought a pair of their flatform heels and I know that she had to go a size up because they squeezed her toes and heal. But it all depends on the style of the shoe and the shape of your foot to be honest.
Order your size and have a look at their return policy. I'm sure you could send them back if they didn't fit and exchange them for a bigger/smaller pair.
I'm not the type of person to wear those shoes but I will admit that they are really nice and if I wore that type of shoe, I'd probably have to get myself a pair. As for sizes, places like Schuh sell them so you could always go in and try them on and get an idea of what size you need.