OK, rule of thumb - that chip, dissipating the power it will to generate the power you're asking, will not need any heat spreading at all. It'll be within a couple of degrees of the PCB temperature. Work out the power it's dissipating (you've got the efficiency curves), and the Tja numbers. Remember the diode will be generating heat too. That'll give you the degrees above ambient.
So, how are you going to control the PCB temperature?
^^^ is the real question. And it's not answered by the 'how much copper area do I need under the chip' question.
(Last I heard, you were doing automotive things. In some automotive situations, you're hard mounted to a lovely great heatsink... In others, you end up sandwiched between a hot plastic body and a carpet, with dreadful heat paths and stagnant air insulating you. Whether your board is hard-mounted to a case or shock-mounted, will make an order of magnitude of difference to your heat path. Whether there's airflow or not, likewise. Since we can't guess, and you won't say, then you're not going to get firm answers, or even firm rules of thumb.
It's entirely possible your design is easy, and thermals can be trivially managed - but, without information, it's hard to guess.)