I pulled this question from the comments, and it's a good one.
Everyone is always seeking a "formula" for success in this business. Years ago it was, "get ten years experience in small or medium markets and then start applying to big markets." Well, these days, that theory is long gone. No rules anymore. Big markets? Experience not necessarily necessary.
Back to the original question: it's not the size of the station at which you start, but the quality. If you truly want to be a network correspondent (and you ought to dig out my post on how grueling these jobs are) then it doesn't matter where you start but what you learn.
There are entry levels jobs in small and medium markets, and occasionally you hear of someone (usually a pageant queen) who starts in a major market. When choosing your first rung on the ladder, step carefully. If you're going to a station that will make you a one man band who chases the scanner, you're not gonna learn much. But if you get a first job at a shop with experienced photogs and veterans who can mentor you, you'll learn ten times as much.
Remember, a package is a package, whether you do it for the network or in the smallest market. Good writing is good writing. Good TV is good TV. The location doesn't matter. What you learn does.