First you need to make sure that your firebox can accomodate the insert(s) you're considering. Also make sure that the fireplace surround provided by the insert manufacturer will cover the front of your fireplace in such a way that it will look right. It should not look like a retrofit. It should look like it was there from Day 1. Carefully visualize how the surround will fit around all sides of the firebox (and below it also, if your firebox is raised above the hearth level). I think you should get the largest insert your existing firebox can accomodate, because it will look so much better. Remember you're putting a firebox inside an existing firebox, so the result will be a smaller firebox. It won't look right if it's too small and skimpy. Assuming you are considering a direct-vent system, the only one worth considering, their btu ratings often imply they can heat a whole small house, but they don't because the heat can't circulate all over the house. We recently got a Jotul gas insert in our 15 by 25 foot living room. We use the remote thermostat because otherwise the area close to the fireplace, the sofa is about 10 feet in front of it, can reach 85 degrees. But the next room gets almost no benefit from the fire. We haven't bothered to use the insert's blower yet. That might help distribute the heated air some. You should also be prepared for a WHOLE DAY process, for the installation of the insert. You'll need first to have your chimney cleaned, and you'll need a special chimney cap that is compatible with the insert. I hope these comments help. I think it's worth doing, but it's not exactly straightforward. Good luck.