Kitchen sink size and what really is the difference between expensive sinks and cheap ones?

Hi there,

My first time on this site. I want to install a square flush mount sink but am unsure as to what size kitchen sink I should install in an island bench which is 2900 long and 900 wide

and

I could spend up big on an Oliveri or Smeg for eg, but do I need to?

I have seen sinks on ebay more than half the price of a smeg or oliveri.