An employer who asks this question is probing to determine if you have a particular comfort level about the size of company where you work. Your resume provides some indication of the size of the companies you’ve worked for in the past, even if the company names are not recognized by your interviewer. Tell the employer that you are concerned about the quality of the work you do, and the effectiveness of the people who work around you. Except in the case of senior level hires, you’ll be working with a relatively small number of people whom you’ll see every day, so the size of the firm doesn’t matter. Certainly there are some positions where the size of the firm does matter, but it is always best to convey that you know where you want to work. Tell the employer: “I’m applying here (or seeking a promotion) because this firm fulfills my criteria for where I’d like to work.”