Isn't the sizing of markets something of marketers need to do, all day long? You can't blow $20k for a Gartner report every time you imagine a new market. You need to be able to make some ballpark estimates.
(Barbers cut the hair of half the population (the male half) around every two months. The population of San Francisco is about 750k. A haircut takes about 30 minutes, and about 50% of the time a barber is cutting hair. This gives about 8 cuts per day, or about 60x8 = 480 haircuts per every two months. So to service 750k/2 of the population you need about 750k / (2 x 480)) = about 781 barbers. Or roughly one out of a thousand people is a barber.)
These 'stupid impossible questions' have a long history. http://en.wikipedia.org/wiki/Fermi_problem
(Barbers cut the hair of half the population (the male half) around every two months. The population of San Francisco is about 750k. A haircut takes about 30 minutes, and about 50% of the time a barber is cutting hair. This gives about 8 cuts per day, or about 60x8 = 480 haircuts per every two months. So to service 750k/2 of the population you need about 750k / (2 x 480)) = about 781 barbers. Or roughly one out of a thousand people is a barber.)