An engineer told me this this rule which was pretty easy to remember:

If you had a 25 mm X 25 mm square, and you stuffed it with 25 nm half-pitch cells, you would get a total of 25 X 10^10 cells (250 Gb or 250 billion cells).

It was the so called "25-25-25-10-10" rule. The lithography scanner field is 26 mm X 33 mm. And of course, real chips would not be stuffed full with dense cells, there would have to be some logic, etc. We already have tens of billions of transistors on a chip today, so it looks like we are quite close to (if not already at) the limit of profitability of the current semiconductor manufacturing infrastructure.