Smallest central heating system

My neighbour has a small flat. I mean tiny. I would estimate the whole thing is about 6 metres by 7 metres. Now, everytime I go round there in cold weather it is swelteringly hot and he has all the windows open. Then I noticed he had central heating. I always thought that central heating isn't used in very small properties as no-one makes a boiler small enough. I assume he has a combi as there is no cold tank.

So, what is the smallest sized property in which you could effectively and efficiently use a gas-fired radiator central-heating system?

has he not got a room stat? and what about rad water temp? ours you can turn it down on the boiler

are you sure you are not walking in to your local sauna by mistake?

Are you sure it's a flat and not inside a oversize combi boiler?

There's really no lower limit. The output can go down to just a few kW in continuous use - below that they just turn on and off.