Jeff Leonard wrote this article about WR Ruvell Martin a couple of weeks ago, but it hits right on point about something I've talked around for a while that Martin is the Packers number 2 WR.
Leonard also has a great analysis of the differences playing wide versus in the slot and why starting Martin makes more sense.
Before you disagree and say Martin should be given the slot and not Jennings, keep in mind that nickel backs play closer to the receivers than cornerbacks do, often employing press coverage as a means of defense. 6'4" players clearly would not have the same sense of balance a 5'11" player would because of center of gravity. If a tall receiver is going against the press, his percentage of beating the coverage will consistently be lower than that of the smaller receiver. Matching Martin up against the opposing teams' cornerbacks instead of their nickel backs means that he will typically have more space to work with since cornerbacks tend to keep the play in front of them with a cushion of open field. Thus, Martin would have more open space to move unimpeded and keep his focus on route running instead of beating the jam. With Martin playing wide out and Jennings at the slot position, the positions that both are more appropriately suited for, the Packers passing offense will become more productive.