length is determined by each shooter or owner. The length, or depth of the hammer jaw, and the use of a leather or a lead keeper to hold the flint in place, and the position of the flint as it strikes the frizzen,, as it's held in the hammer is a variable.The shooter adjusts the flint.
If size is a kwandrie whilst purchasing, buy squares. 3/4x3/4,,5/8x5/8,,1/2x1/2.
Some hold bevel down, some hold bevel up,,some find a sweet spot with just that special curve or tip to the stone,,
Remember, it's the frizzen that makes the sparks! The rock is just hard enough, and sharp enough,, to scrap the molten sparks of metal off the frizzen..