I'm going to suggest something crazy. You probably won't be able to estimate the size of your market using the internet. Go to a research library where you ca

I'm going to suggest something crazy. You probably won't be able to estimate the size of your market using the internet. Go to a research library where you can read and copy paid industry and market research reports for free (the library pays the fees.) Bring a USB key and you can download the reports (at least in SIBL in NYC.) I would suggest you do some old fashioned market research by looking at the age/sex/demographics/industry of your target customers and reading market research reports on them and industry reports on your competitors and other replacement products. A replacement product is one that isn't a direct competitor but people can use instead of your product (for example: Gmail and Google calendar is not a direct competitor of a CRM system like Salesforce.com, but people can and do use it instead of a CRM system.) I think you could do all of this with 1 day spent in the library. You can find the same information on the internet, but it will take you a lot longer and you would have to pay for many of the same reports you could get from the research library for free.