A company's market capitalization is the number of shares available on the market multiplied by the price of the stock at any given time. You can find a public company's market capitalization by looking up the company in Yahoo Finance or Hoover's.
For historical market capitalization, I usually use print products. For larger companies, the market capitalization is in Standard & Poor's Stock Market Encyclopedia. For other companies, the S&P Daily Stock Price Record lists the number of shares outstanding at the top of a column listing the stock price for each day of the quarter. If the shares outstanding isn't in the Daily Stock Price Record, check the relevant Mergent manual entry for the following year.
For smaller companies, the "tear sheets" usually list the market capitalization for small companies, and the Bloomberg reports (available from Bloomberg or Lexis) should have it too.