When asked about “salary expectations” in an interview how should I respond?

Answer: You should not beat yourself up for flubbing the question. It is a very hard question, especially when one is coming from a large firm, for two reasons. First, large firm associates are paid under a "lock step" system so the question has probably never arisen for you in the past. Second, it is unlikely that you know anything about the pay scales at small firms and (even if you did) the pay scales at small firms vary widely.
There is no "right" answer, but you may want to - politely - turn the question back to the interviewer by stating an obvious point - he has infinitely more information than you have regarding some key issues. He knows much more about the position, the firm's pay scale, and the benefits that they offer to their associates. What is the firm expecting in terms of hours? Do they pay each associate at your level the same or has each associate negotiated his compensation? What types of benefits do they offer? In order to effectively answer the question, you might say "Well, you have much more information that I do about your pay structure and the value of your benefits package so perhaps you could tell me what you believe is a fair salary for a candidate with my skills and experience?"In this way, you state a simple truth, and in doing so, ask the interviewer to be fair. He does know a great deal more than you with respect to this issue.