They're listed as one group, but there are a few different kinds of defensive back. First, there are safeties and cornerbacks. Among those, there are strong safeties and free safeties, edge corners and slot corners.
There are different requirements for each one.
In general corners will be more athletic than safeties. Edge corners generally (but not always, depending on scheme) will be bigger and longer than slot corners. Likewise, free safeties will need to be rangier and better in coverage than strong safeties, who need to have stouter builds to contribute in the run game.
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Height, Weight, Arm Length, Hand Span
Taller defensive backs are becoming more and more en vogue. Not only do the match up better with the NBA-sized receivers entering the league, but the most recent Superbowl champion featured a secondary full of industrial sized DB's.
However, height is not without its disadvantages. Taller corners have a hard time sinking their hips in their backpedal, and being smooth in coverage. Yes, they can cover larger receivers, but they have a hard time covering tightly. Likewise, heavier DB's have the same problems. They can be physical with bigger receivers, but they tend to lack the quick-twitch athleticism to stay with quick receivers in space.
Long arms are a big advantage for a defensive back. It allows them to play bigger than their frame would indicate, letting corners with the size to be quick, be able to cover area like a larger defensive back.
Big hands are useful when jamming receivers or for snagging interceptions
40 Yard Dash: Like the wide receivers they cover, raw speed is important for defensive backs. Corners need to be able to turn and run with receivers, and safeties need to be able to cover ground.
Vertical Jump: The vertical jump shows two things for defensive backs. First, it shows lower body explosion, and their ability generate power. Second, defensive backs need to be able to leap to defend high passes and be better-able to high-point the ball than the receiver
Broad Jump: Again, this drill shows a DB's ability to generate power.
3-Cone and Short Shuttle: These drills are very important for defensive backs. Corners especially need to have quick feet and loose hips to transition and run with receivers.
W Drill: Deion Sanders once said that he tells kids that if there is one drill every DB should do, it is the "W-Drill". This drill give prospects a chance to show whether or not they have the quick feet and loose hips in transition. Safeties, and strong safeties in particular, won't perform well here, but top-flight cover corners should excel in this drill.
Backpedal Drills: I'm lumping a few drills together in this section, but they are all variations on a theme. Basically, the prospect goes into their backpedal, and then flips his hips based on direction from the coach running the drill. Then, they turn and either run down the field or break at a 90 or 45 degree angle. The purpose here is to first see how good the prospects backpedal is, then how loose their hips are when breaking on the ball. Also, because the drills force prospects to react the direction of the coach, it shows how well they are able to react to what a quarterback does when they are in zone coverage.
Close and Speed Turn Drill: This final drill has the DB start out with a backpedal. Then he has to plant, change directions and drive back toward the quarterback, before having to transition again and play the ball down the field. Like the previous drills, this one shows the prospects' backpedal as well as their ability to transition out of it suddenly. Then it shows how they can turn and run and how well they are able to track the ball going down the field.
How It All Ties Together
Teams need to figure out which defensive backs are safeties in the NFL, and which are cornerbacks. There are corners playing in college who can cover college receivers, either due to similar athletic abilities, or because college rules allow corners to be more physical than in the NFL. However, those guys might (or even will) not be able to stay at corner against the best receivers in the game.
Likewise, there are few things as valuable to a defense as a safety who is able to play center-field without letting passes go over his head. Having a safety with that kind of range and coverage ability gives defenses the flexibility to play with more men close to the line of scrimmage to defend the run, or to bring extra pressure on blitzes.
The measurable events and the position drills defensive backs are designed to separate the corners from the safeties, the outside corners from the slot corners, and the free safeties from the strong safeties.