I'd be careful about believing any numbers, and just because some big market research company once said it doesn't mean it's true. Certain things spring up as b

I'd be careful about believing any numbers, and just because some big market research company once said it doesn't mean it's true. Certain things spring up as being the truth after they are quoted enough, e.g. the Internet traffic doubling 18 months (http://www.lightreading.com/document.asp?doc_id=18702 http://www.lightreading.com/document.asp?doc_id=18702). In my experience the best way is to analyse a market in several different ways and get bounds on the market size. E.g. for baby monitors in the US, the market size must be less than the total revenue of all the companies that make them, and the units sold per annum must be less than the number of babies born per year in the US. You then may find one company that claims to have shipped over a million baby monitors in a year (e.g. inside their SEC filings or even marketing material), which is therefore a lower bound. A combination of google, asking people (e.g. either industry experts or in your local shop ask the shop assistant roughly how many of those do they sell a day) and you should be able to get to an order of magnitude of an established market place.