First, there are many important steps you'll take when investigating any franchise but the three most important are: 1) contacting the company directly to receive its franchise information including the federal mandated disclosure document (the FDD), 2) carefully reviewing all of the information you receive to make sure it sounds like a business you are interested in, and 3) calling a number of the existing franchisees to learn much more about the franchise business from their perspective.
Second, the size of the franchise system isn't nearly as important as other factors, unless it is a new brand. You should focus on a franchise that has at least some track record of success with other new franchisees if you want to minimize your risk.
Third, the fit is critical. This is the most common mistake that people make -- they fall in love with the product or service of a franchise and purchase it only to figure out later that the business is not a good fit for their skills or temperament.
Each of these topics is covered in much greater detail in articles that I've written for Entrepreneur and that are archived in the Franchise section of their website so you can get much more extensive answers there. Good luck in your search.
Jeff Elgin has almost 20 years of experience franchising, both as a franchisee and a senior franchise company executive. He's currently the CEO of FranChoice Inc., a company that provides free consulting to consumers looking for a franchise that best meets their needs.