During the development process of Ophena magnetic stirrups, we decided to offer the stirrup in one size only. We chose the dimensions carefully and are really happy with the results. They fit women, men and children equally. They look sleek, but offer enough space for common foot sizes (we offer the insoles from EU 32 to 46). Some stirrups are bigger to decrease the risk of getting caught, but Ophena mitigates that risk due to the open design.
To be specific, this is the size of the tread (top-view):