In theory, a larger shell can hold more shot and a bigger powder charge for increased power and range, but at the cost of increased recoil. In practice, shell length is largely irrelevant, and the powder charge and payload are the numbers to pay attention to. There are plenty of more important factors than the chamber size when it comes to picking the best turkey load (and not all guns are chambered for magnum 3.5” shells). I’ve killed plenty of birds with a 2-¾” load, but for me, a 3” shell is just right.