Well ultimately you'll be the judge of whether a market is large enough--it just depends on your goals really. As far as actually quantifying it though, there's

Well ultimately you'll be the judge of whether a market is large enough--it just depends on your goals really. As far as actually quantifying it though, there's several methods. I personally think the easiest traditional way of doing it is called the break down method. This involves taking a total population, and then applying various percentages to whittle that number down until you got your target market. To illustrate, lets say you're selling an iPod hookup for cars. You're targeting US only so you have the total population n. You also know that x% of people drive cars and that y% of people own an iPod. To get your market size you multiply n by x and y--voila. A less traditional way of doing all of this involves a little facebook hack I've been tinkering with. If you go through the process of creating an ad on their network, it will give you the number of users for the various filters you place on it (you can specify your demographic by location, age, sex, keywords, education etc etc). In certain areas of the world the % of people on Facebook is so high that you can basically use this figure as a solid estimation. I haven't tested this for accuracy but it works well enough for most purposes.