Product Management and Marketing are intertwined, and this means that they collaborate on new product proposals.
One of the stalwarts of this is some back of the envelope market sizing. There are lots of guides and best practices out there on how to do this quickly and cheaply (especially if your product isn't big enough to be covered by Gartner or Forrester,) that I will not describe.
That said, there is the tendency to Go Big on market sizing numbers to get attention. The Dude knows this, and understands. He has been meticulous throughout his career to not over sell, because he is terrified of having to answer why his product missed the targets.
But today, in a product approval pitch (not his), the Dude heard a peer product manager say that the TAM (total addressable market) for a widget was an eye popping $2.2T USD
That was not a typo.
15% of the US GDP.
There are no $2T product opportunities. Get over yourself.