5KW or less
Any stove rated above 5KW will require an air vent to be installed in the wall leading to outside. This is to allow enough air to enter the room for the stove to operative safely and effectively - it's the law.
Many customers choose to have a stove rated 5KW or under to avoid such a vent. Further to that, 5KW is often MORE than enough to heat an average family room. If you would like to check for yourself, a room size/stove output calculator is available.