Even though it's only listed as two positions on the program, there are a bunch of different types of players being tested Monday on the third day of on-field workouts at the 2014 NFL Combine. Teams that run a 4-3 defense (such as the New York Giants) have to look for a completely different set of players as a team running a 3-4 front.
In general, the 4-3 defense needs better athletes across the front seven. The defensive line is asked to both stop the run as well as rush the passer, while the linebackers need to have good range, cover, clean up in the run game, and be athletic enough to blitz.
Height, Weight, Arm Length, & Hand Span
All of these are important for defensive linemen, but in different ways for each position along the defensive line.
Big hands and long arms are a plus wherever a defensive lineman plays, allowing them to get their hands on an offensive lineman first and either control him or shed the block.
Height and weight vary more widely for defensive linemen than pretty much any other position. On one end you can have 3-techniques like Aaron Donald at 6-feet, and at the other you can have defensive ends like Devin Taylor who are 6-7. Likewise, speed rushers on the edge can be as light as 250 pounds, while 3-4 nose tackles can weigh 350 pounds. Generally, there is a prototype for each position along the defensive line.
Like the Height, Weight, Arm, and Hand measurements, what teams are looking for here varies widely. Nobody is going to care if a 350-pound nose tackle doesn't have good numbers. What they care about is whether or not he can occupy blockers and collapse the pocket.
Athletic pass rushers, however, will need to test out well here. These guys need to be quick and powerful to get past offensive linemen and get to the quarterback.
10-yard split, Broad Jump, Vertical Leap: I'm lumping these three together, because they all help to illustrate a prospects lower-body explosiveness and his ability to generate power. To me, these numbers are much more important than how many reps they have on the bench press or how fast their 40 is.
3-cone drill and Short Shuttle: Defensive linemen need to be able to have short-area quickness. Edge rushers especially need to be able to bend around the corner to get to the quarterback.
3-Bag Drill: This drill has the prospect shuffling along the line of scrimmage, alternating between tapping bags on the ground and punching bags standing up. Teams are looking to see how players can move, bend, get out of their stance, and how violent their hands are
4-Bag Drill: Similar to the three-bag drill, this one has four bags on the ground. The prospect starts out in the middle of them, goes over the bags to one side, then back the other way, then goes back along the line bending down and hitting each of the bags as he goes. This drill shows which prospects have the quick feet to be able to work through the trash at the line of scrimmage, movement skills in space, and bend/flexibility.
Pass Rushing Drill: This drill shows which prospects can bend and turn the corner to rush the passer. They go from the left and right sides, and have to get around two tall bags before getting to the "passer".
How It All Ties Together
When you hear scouts talk about pass rushers, one of the most common phrases you hear is "Converting speed to power". That refers to a rusher's ability to get low in their stance, explode off the snap, then use their burst and leverage to move the offensive lineman backwards, then discard him on the way to the ball carrier. To do that, they need to have good flexibility in their ankles, knees, and hips; strong lower-body explosiveness; and the ability to bend and play with good pad level (ie, lower than the offensive lineman's). These are what scouts are looking for in the measurables and position drills.
Height, Weight, Arm Length, Hand Span
While there is no such thing as an "Ideal" linebacker size, they don't vary nearly as wildly as defensive linemen do. Generally, scouts want to see linebackers are between 6-2 and 6-5, and 230-250 pounds. They have to be big enough to deal with blocks from offensive linemen or tight ends, and tackle power backs. However, they have to be athletic enough to cover tight ends, running backs out of the backfield, and rush the passer.
Like the defensive linemen, however, long arms and big hands are a definite plus for linebackers, and for the same reasons.
I'm just going to go ahead and refer you back to this part for the defensive linemen, and for the same reasons.
The biggest difference is that the full 40-yard dash does matter for linebackers. For the most part the front-seven plays in a 10-yard bubble, but sometimes linebackers will, be asked to turn and run with tight ends, running backs, and even receivers.
The pass rush and three-bag drills are the same as the defensive linemen, and are important for the same reasons. There are some linebacker specific drills and that's what I'm going to concentrate on.
Pass Drop & Hip Rotation: This drill has the prospect dropping back at a 45-degree angle, and flips his hips three times and changes direction while dropping back before making one break back towards the coaches to intercept the ball.
Wave Drill: The same drill as the offensive linemen run. Scouts want to see how fluid the linebackers' movement skills are, and get a look at their ankle, knee, and hip flexibility.
How It All Ties Together
Linebacker is one of the more difficult positions to scout. That's fitting, since it's something of a mirror to the running back position. Like the RBs, linebackers depend on their defensive line to keep opposing linemen off of them so they can make plays. There's also a bunch of different types of linebackers.
However, the ones that go high in the draft and make the most money in the NFL can: A) Rush the passer, B) Play with range, and C) Cover tight ends and running backs. As offenses seek to spread defenses out, defenses need athletic linebackers who can affect the passing game (either through coverage or pass rushing). That is the goal of most of the position drills
They still need to be able to clean up in the run game though, and that means the ability to fight off and shed blocks, and stop a running back when they make the tackle. That is where the lower-body explosiveness numbers come in.
Finally, linebackers need to be able to recognize plays and react to them, which is part of the reason why most of their drills involve them reacting to the coaches' directions.
One more day of workouts to go, with the defensive backs taking over Tuesday Brace yourself, Richard Sherman comparisons are coming.