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power integrations have lots of app notes and they show pictures of heetsinks used for their power supplys.....you can also use thier software to design a flyback and i think it gives you heetsink aswell.
But anyway, i try to answer you....
Your heatsink depends on your enclosure....is it vented or not (got holes in it)...is it made of metal or plastic.....do you have fans also to cool? in what max ambient temp is your psu needed to operate.
So i bring more questions to your door...but is needed.
Eventually we will get on to the power dissipation suffered by your fet.....and then we select heatsink with correct thermal resistance....but i will not talk rubbish to you...most people dont do this...but just rely on experience or educated trial and error...or look at app notes , other designs, to see what size they used.
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but by all means crack open some laptop power supplies to see how they heatsunk.
Also, what proportion of time is your flyback on max load?