This issue is most just a matter of size. It's literally a matter of what will fit in a particular size lock and allow it to function appropriately. This is because there is a limited about of space between the front edge of the top jaw and the face of the frizzen when the cock is at half cock. When the cock is at half cock, the flint needs to stick out far enough from the jaws that the top jaw or jaw screw doesn't hit the frizzen when the cock swings forward and down. That's to say that the flint has to hit, obviously, or it's not gonna work. The flint also needs to be short enough that it doesn't touch the frizzen when the lock is a half cock and keep the frizzen from closing fully.
I might seem like you could put a longer flint in the jaws and then just put it on full cock when priming it, so as to have clearance for closing the frizzen. But it's not a safe solution. The half cock is designed for holding the cock while you are priming the pan, and putting it on full cock while you are priming the pan is asking for trouble. In the least, if it is accidentally released it can cut you. And of course it may also cause an accidental discharge. Furthermore, on some locks, the angle at which a long flint like that will hit the frizzen will be wrong, and the strike can sometimes just knock the frizzen open without making sparks. So the flint really needs to the right length, within certain minimums and maximums.
In a pinch you can sometimes use a smaller flint on a large gun by wedging a piece of hardwood in behind the flint, so as to push it out further. Conversely, you can sometimes use a larger flint in a small lock by knapping a notch in the back side of the flint so that it can slide a little further in. (This is easy to do by chewing a notch with the corner of a pair of plyers). But we are only talking about gaining a little wiggle room either way. Much more or less than what really fits and then the flint won't work. That's been my experience, anyway.
