But fire doors are the size of normal doors. They just tend to have giant warning signs on them, are sometimes painted red and tend to have "push bars" instead of handles.
Door size has very little to do with fire door "affordance".
...and in your example, you said a normal sized door next to double-doors. Yes, there are other signals indicating emergency exits. But I was assuming, from your example, that the only piece of information we had was the door size. My point was that most people would then look for other signals to indicate what was going on - such as giant warning signs and push-bars.
Door size has very little to do with fire door "affordance".