Whilst I'd tend to agree with Junkyard, published stove output figures are hardly as accurate as those for a conventional oil or gas boiler.
Moisture content, species and size of logs as well as how you load it can really affect the real world performance. And suitability for a given volume depends on levels of insulation etc.
The first stove we had was a hunter hawk (5 kw), which was supposed to be perfect for our size of room. In reality, the firebox with the multi fuel grate in, was too small and it didn't pump out as much heat as it should have done - I only learned later that I should have removed the grate!
So, I'd be less concerned about published figures and go for one you like the look of and learn to tweak it's firing (get a flue stat) to make it work for you.