How do I know before I call a fireplace installer if the flue is large enough?

I am thinking about having a wood burning stove installed in a space where an old ceramic heater was. The old ceramic heater was installed in a space that looks like a fireplace, with the mantel and everything. I want to know before I even bather with a pro, if the flue can handle it. If it can't I am not even going to bother because it would be too big of a renovation. Any help would be appreciated.