As some have mentioned above, a long stride is a positive factor with two major pitching components if done correctly: first if you stride long, you develop more momentum (weight shift) onto the front leg, that if sequenced properly will transition from the legs, hips, to torso, and ultimately into arm whip (arm speed), this is what creates velocity, and the difference between what some call a "free and easy" versus "max effort" style of pitching. The second benefit is what some call "visual velocity" whereby the closer you are to the hitter at ball release, the faster the ball appears, or gets to them (think Randy Johnson or Aroldis Chapman).
One big caution to the above,... a long stride can be a bad thing! If the core strength, hip flexibility, or even ankles are not developed / or strong enough to support the long stride, you actually have a negative effect. Things to look at are: where on his front foot is he landing?..., does his front leg brace up and not leak into ball release?..., and what angle is his back in relation to the ground at ball release?... If any of these are lacking then you will see negative results, and a shorter stride is in order, until those issues can be resolved.
Also, I was unclear in your post if his stride length is being measured without actually throwing a ball...if so, you may want to update us on the results after an actual bullpen is thrown from an elevated mound, at game speed.