Roughly speaking, the flint should be as wide as the #### it fits in. If it is too wide, it will probably hit the barrel at the end of its swing. Too small and it will barely or not reach the frizzen. If they are too thick or if they have a high peak in the middle, the top jaw of the #### will angle upwards and squeeze the flint out as you tighten the screw.
With this last statement in mind, the height of the shoulder that the back of the top jaw rests on, governs the angle of the top jaw. If you get stuck with some really thick flints or ones with high peaks in the center, you might try putting a piece of wire around the #### as a shim on the shoulder, so that the top jaw angles down.
cheers mooncoon