Well it doesn't. There exists something called thermal resistance which always comes into play. This is why you would need a thermal compound on eg. a CPU.

Just thinking that there is no thermal resistance would nullify all calculations made. Of course if you just want an estimate and wildly overdo the actual radiator go ahead.

It's been some time since calculating these in school, but I'd say you would need the heat transfer coefficient of copper/square meter and keep going from that.

And no I am not doing the calculations for you