Answer: You have asked a good question, but before I attempt to answer it, I must ask you a very important question: How do you know that you will want to be at a small- or medium-sized firm for the bulk of your career? You are just starting out ... I submit to you that it is nearly impossible to know for certain what you want to be doing five, 10 or 20 years from now. Who knows? You may not even be practicing law 10 years from now!
Now, on the other hand, I do think it is a good idea to keep all your options open. I believe that the best way you can position yourself would be to take a job in a major law firm if you have that opportunity.
Of course, there are different types of major law firms. For example, if you go to work for a large, national insurance defense law firm, more than likely you will not be a viable candidate for the firms that do work for the Fortune 500 companies. Similarly, should you choose a large medical malpractice firm then you should be setting your sights on firms that have that type of focus, rather than the firms which represent Corporate America.