Does that mean his first NFL roster will look the way he wants it to look? Does that mean there aren't exceptions? No and no. But it's a belief that Kelly holds dear, especially considering that he was willing to admit it on Wednesday, and one the scientific coach has already enacted.
"You have to adjust to what you have. No one is starting from Square 1 and saying, 'How do we build a perfect defense, offense, and special teams?' " Kelly said to a roundtable of reporters at the NFL meetings. "And you don't have 100 first-round draft picks, either, so you can say, 'Hey, I really like that guy, but he's gone.'
"So you always have to make adjustments to what you do. But we want taller, longer people because big people beat up little people."
If that's true, Kelly's offense as currently constructed is in trouble. But his facetiousness aside, Kelly has a point if you're looking at some of the NFL's better defenses in recent years. Last season, the top four defenses - the Steelers, Broncos, 49ers, and Seahawks - ran some version of a 3-4 scheme.
Defenses with odd-man fronts are typically larger than 4-3 teams, but those four squads had some of the longer players in the league, especially on the edges.
The Seahawks ran a hybrid defense and could be the template for the Eagles. Seattle defensive coordinator Gus Bradley was the runner-up to Kelly for the Eagles job, and defensive coordinator Bill Davis is expected to employ a system with some of the same traits. Kelly, who started scouting "longer" players when he became head coach at Oregon in 2009, may be looking back to the Pacific Northwest for more than just schemes.
The Seahawks defense, which led the league in points allowed, was significantly larger (averaging 6-foot-2 and 252 pounds) than the Eagles (6-1, 242). Seattle was huge in the secondary with cornerbacks Richard Sherman (6-3, 195) and Brandon Browner (6-4, 221) and safety Kam Chancellor (6-3, 232).
The Eagles were long at the cornerback spot as well with Nnamdi Asomugha (6-2, 210) and Dominique Rodgers-Cromartie (6-2, 182), proving that measurables matter only so much when evaluating talent. But all things being equal in terms of skill and mind, Kelly is going to take the 6-5, 265-pound outside linebacker over the 6-1, 265-pound one.
The Eagles have added some size to that spot, signing free agent Connor Barwin (6-4, 268) last week. They replaced Asomugha and Rodgers-Cromartie with the similarly sized, but more motivated Cary Williams (6-1, 190) and Bradley Fletcher (6-0, 200). Safety Kenny Phillips (6-2, 217), if healthy, would tower over Kurt Coleman (5-11, 195).
Kelly, as he mentioned, can't remake the defense in one offseason. So defensive end/linebacker Brandon Graham (6-2, 265), linebacker Mychal Kendricks (5-11, 239) and cornerback Brandon Boykin (5-9, 182) will remain. It helps that all three have upside, too.
The Eagles could draft a prospect with the No. 4 overall pick this April who would meet Kelly's preference for long defensive players. Maybe they won't go with a pass rusher, but Oregon's Dion Jordan (6-6, 248) and Brigham Young's Ziggy Ansah (6-6, 270) would seem ideal fits.
Both are fairly raw. Both could pass for basketball players. And both have long arms - Jordan's measure 337/8 inches; Ansah's are 351/8.
Kelly doesn't seem as particular about the size of his players on offense, especially at the skill positions. He seems to favor those with speed. Go figure. But he has said before that he likes his quarterbacks to have large hands.
Kelly spent much of the 72-minute interview bobbing and weaving from attempts to pin him down on certain subjects.
But the coach admitted that he is intrigued by how nutrition, conditioning, and sleep affect a player's performance. "An elite athlete needs between 10 and 12 hours [of sleep] a night," he said.
The bigger, and better-rested, the better.
Contact Jeff McLane at email@example.com. Follow on Twitter @Jeff_McLane.