It should be pretty easy to come up with a rough estimate of how big of a market there is for your product or service. If you do believe you have an idea for a product that lots of people would want, by all means pursue it. As far as advice to help you out on your path, you could do worse than reading some of pg's articles: http://www.paulgraham.com/articles.html

First thing is ask yourself is this idea what users need? Is this idea a want or a need? Do you have any competitors that are doing the same?