There are two things that determine stove size; required heat output for the room, and the actual or potential size of the existing fire place.
To calculate the required heat output divide the cubic capacity of the room (width x length x height) by 500 if working in feet or by 14 if working in meters. This gives you the kw output needed of a stove and should be your main guide to choosing an appropriate stove. Modern stoves have optimum temperatures at which they need to operate to give their true efficiency. Therefore just because you have a large fireplace it does not mean you should fill it with a large stove. If you consistently run your stove at a lower temperature you run the risk of tarring up your flue and your stove will not operate efficiently also causing consistent blackening on the glass.
Secondly – will the stove fit into my fireplace? The space around a stove is important to allow the heat to escape to the room and avoid damage to the stove. It also looks better if the stove and fireplace are kept in scale.