Measure the width of your frizzen. With the lock at half cock, measure from the jaw screw to the face of the closed frizzen. Subtract 1/8". If that distance is greater than the width of the frizzen order a flint that is the width of the frizzen by the width of frizzen + 1/8".
You can also just use the width of the frizzen as the flint size. So, if your frizzen is 7/8"wide and the distance from the jaw screw is 1 1/4", order a flint that is 7/8" by 1" or a 7/8" square. Its easier to move the flint forward for length than to remove width from the flint. You can always add a small strip of leather lacing to move the edge of the flint forward or under the back edge of the flint to give the edge more of a scrape than impact.
When you order flints, be sure to state that you want flints with the top and bottom to be flat. You don't want the humped top flints.