And why 5'9 CBs will struggle to succeed in today's NFL.
There is a growing change in the NFL, perhaps one that some of you have noticed, its more than just a change in the rules, or emphasis on protecting players, its more than a growing number of tall incredible athletes coming into the NFL at the Receiver position (both WR and TE), it's also the growing complexity of many Offenses and the increased talents at the QB position.
Imagine having to plan on playing the Panthers, and their rookie sensation Cam Newton. That's a tall order for a CBs to handle, because not only do they have to try and cover WRs against a QB who can toss it 50 yards downfield with accuracy, but they have to concern themselves with being the last line of defense against Newton tucking the ball in and running it down their throats... and at 6'5 and 250 pounds, that's one big body coming at you.
As for the size changes at the WR position, in 1989 the average height of WRs in the NFL was 71.0, in 1999 it was 72.3, in today's NFL it is 73.1. Most teams now have at least one 6'3+ WR, Green Bay has Jordy Nelson, the Bengals have AJ Green, the Falcons have Julio Jones, the Texans have Andre Johnson, etc. it is becoming more the norm to see a team with a tall and fast deep threat WR, rather than the exception.
That's not considering the ever growing number of Gronk and Jimmy Graham type TEs, the majority of teams have at least one 6'5 250 pound TE capable of getting downfield and causing mismatches, if not more than one. And more will be joining them after this year's draft (IE - Coby Fleener).
There's no denying that size can give a receiver a major advantage. Good WRs with excellent height cause great matchup problems for CBs, especially in the red zone. The ability to make plays on the jump ball or adjust to poorly thrown balls is the forte of receivers who possess great size. The rules emphasis favors the taller, stronger, WR who can get away with pushing a CB out of the way or interfering with him despite the CB having position.
The days of the smaller receivers being in vogue are gone. Undersized WRs must be difference makers in the return game, be extremely tough and be capable slot receivers (IE - Welker). They must be physically tough and have the ability to separate, get open and make the first down or they will have short lived careers in today's NFL.
The main reason why I believe BB drafted Dowling as early as he did in last year's draft was his SIZE as well as ability, in recognition of this trending change in the NFL. The reason why the NFL had more teams passing for 5,000 yards than ever in one season last year was because of the changes in the NFL that have been occurring since 2006.
All these factors make it increasingly difficult for a smaller CB to be successful in today's NFL, and it is the smaller corners, combined with a lack of a Pass Rush, that I feel was the primary reason for the Patriots poor defense against the pass last year, despite facing some of the NFL's worst QBs and worst offenses during the latter part of the season.