First parameter is width of frizzen. The flint ideally should be the same width. Second is distance from tip of flint in the jaws to the frizzen at half cock...should be some clearance... about 1/8" clearance is normal. If you have a choice, choose flints that are flat on the top rather than peaked, and about 3/16" thick. In my rifles, I get faster ignition with the flint installed bevel up rather than bevel down, especially in big locks.
English black flints work just as well as French amber flints, in my experience, though I use both.
Knapping flints: if your lock misfires, it is likely that there is a spot on the flint that is too dull to strike a spark, and is just pushing the frizzen out of the way without sparking. Find that spot and remove it by knapping, supporting the bottom of the flint with your index finger as you strike down and toward the butt with your knapping hammer. I use a 2" x 3/8" square length of copper installed as a "T" across the end of my turn screw. As your flint wears from shooting and knapping to restore the edge, it will become shorter, and it will be necessary to loosen your top jaw screw and move the flint forward. Sometimes it's good to support the back edge of the flint in the jaws of the cock with a small twig.