a concave mirror can be represented by placing a convex lense or normal magnigying glass (that we all agree will indeed start a fire when used appropriately, or as some of us have done, inappropriately) in front of a mirror. If you are positioned at the proper distance, your reflection will be magnified. If you stand farther away, other strange affects happen. It is all about positioning things at the focal point.
This is just like a satellite dish. The signal is almost in parallel lines hitting the concave inner side of the dish and are concentrated on the feed horn when the whole shebang is aimed and set up properly. If the same dish were aimed at the sun and lined with foil or otherwise made to be optically shiny, there would be a focal point where the concentration will be at a maximum, where things could be burned, cooked or otherwise heated. This solar focal point is probably just before the feed horn because the sun's rays have travelled farther. They are not any more parallel because the sun is so much larger, in fact it covers more sky than any satellite, so the rays are not more parallel, but they will focus a little closer than where the feed horn's center is.
Of course as you move farther and farther from the dish, the reflected rays will be farther and farther dispersed. As they approach the feed horn position they are more concentrated, but after passing it they are spreading back out. At twice the distance to the feed horn they become more dispersed than the normal unreflected sunlight falling around the dish.
I have no idea what the focal distance for the originally described mirror would be, but it would be somewhere between the distance that your eye would be in perfect magnified focus when looking into it and half that distance. If the mirror were to be aimed at the sun and an easily scorched object were to be placed about 2/3 the distance of a self seeing eye, I would expect it to start to smoke in short order, unless the object shadows too much of the mirror.
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