I see it come up over and over again, like this is a 2$ billion market, we are in a 3$ billion market, etc...
Where do you find this data? I've been searching and it's really hard to google something that makes sense.
I did a bit of searching and for the US you have access to various statistics out of Census: http://www.census.gov/econ/census07/www/get_data.html. The data is from 2007 with 2012 being the next census year. You can drill down in to industry and sector to view sales data - for example Retail Sales.
Unfortunately these reports are expensive to purchase, but often you get the market size quoted in news reports. Some canny searching in Google News might get you figures: include Gartner/Forrester/IBIS in your search term and you might get lucky.
You could also try and find somebody at college/University who has a login to the databases on the library web site. Sometimes you get lucky and can to the IBISWorld database and a selection of reports are available.