In addition to everything mentioned, I believe talking about speed and length is critical. (as well as big skill, especially for coverage linebacker.)
For example you have Ishaq Williams, who is damn near six-and-one-half feet tall, but runs with wide receiver speed, playing (what I believe will be second rush OLB, (Cat).
The most interesting case is Prince Shembo, who moved from inside to outside CAT to DOG. He is in a fight to start at DOG with Danny Spond. Both Shembo and Spond play long. Their arm span is (considerably) more than their height and they can change direction quickly and accelerate with blinding speed. Playing either at OLB for the cover position is like having an extra (very large) strong safety, with an attitude. (When both of those guys, particularly Spond hit during the Spring game, it had a whole different sound!)
As far is ILB, size (body density) is really important because of how important it is for them to defeat offensive guards who come charging at them tipping the scale around 300lbs. I shook hands with Manti Te’o, briefly, and was awe struck how much more developed physically he was than last year. Next to KLM and EJ who are ABSOLUTE BEASTS, I mean BLOCKS OF GRANITE, he looks most changed to me. (I am excluding offensive linemen, because I laugh at the criticism they received after the spring game. They are HUGE, and I will be surprised if they don’t BURRY some people this year. Not only are the starters massive and in shape, you have a whole second line behind them ready to go. I mean it watch out for Nichols and Lombard!)
The other thing nobody has discussed yet about either of the OLB positions is the smarts required. I think you have to be smarter to play Diaco's defense than most, but both OLB positions require players to make a series of decisions almost instantly, as well as rule out what cannot happen in a given play to enhance (correct) anticipation. THAT WAS A BIG PROBLEM EARLY LAST SEASON WHICH RESOLVED IN THE FINAL FOUR GAMES.
OMM (who is one of the sharpest on this site) has a good point about Coach Kelly. Often times he lets the player find his position, instead of starting with a whole lot of preconceived notions. Let me add to that: You can bet that every player is evaluated as to body type. This is not limited to height and weight, or 40-yard dash speed. He is looking for shoulder height, arm length, hip and chest width and so forth. In addition, the ratio of fast twitch to slow twitch muscle fiber, (explosive speed to endurance), thigh to hip ratio, lose to tight jointed, ability to put on weight, etc., is all evaluated, often before a prospect is offered.
I was struck during the spring game, how the body types of players at different positions fit their positions for the first time in a long time, and how the builds of some veteran players have changed in this last year!
