Major companies routinely try to "size" the markets in which they compete, and the results are often public. So look online at the 10Ks and other filings of publicly traded companies in your field, and listen to Webcasts of analysts' conference calls.
At census.gov, you can find info on the size of your potential customer base, be it U.S. businesses or consumers.
Associations for various industries publish detailed reports on their sectors. They usually include estimates of how fast the industry is growing and an analysis of major trends.
ACNielsen (ART)(nielsen.com), Forrester Research (FORR) (forrester.com), and Gartner (gartner.com) publish extensive research reports on technology and consumer markets. Some are free, and free summaries are often online for the others.
Return to the BWSmallBiz October/November 2009 Table of Contents