I recently purchased/installed a Hearthstone Clydesdale insert (which I'm pretty happy with) and the window is fairly large (about 20in wide x 12 in high) and you can purchase a screen which allows you to leave the door open safely.

If you do not want to install an insert, there is an alternative, but you will need to do some searching, because I do not have specific information about it. The device I have in mind, invented by a physicist, is a specially designed iron crate into which the wood is stacked in a certain specific way. The advantage of the system is that much less heat is lost up the chimney than with a conventional system. So it is much more energy efficient.

If you cannot find anything about this from searching the web on your own, go to hearth.com. They have a forum of fireplace experts who might be able to guide you.