OK, so maybe most everyone knows more than me but having gone through your stage only a year or so ago let me add a thought or two.
From what I have seen the measurements given tend to represent the width of the flint and that was confirmed recently by an outfitter recently when I purchased a few new ones. It seems like once the width is established they tend to be "napped" in a length that is proportional to the width (but not the same as).
I do not want a flint that is wider than the frizzen, in fact I want one smaller than the frizzen width. All of the frizzens on my rifles have 1 inch faces so I look for flints that are sold as "3/4 inches", but the measurements are rarely exactly correct.
I have purchased some from ToW as well as on ebay. I usually ordered several at a time and it never fails that one or two are odd shapes and don't fit well, and some even too long or two short. I think that's the reality of bulk purchases. Now, I only purchase from a local outfitter and I select each one based on width, length and shape. I don't care for those with big humps or irregular edges.
Another discovery about flint size is that one needs to be careful the inside edge or point does not hit the barrel when fired. You can put it in the jaws, offset to the outside if necessary. And you can align it so the edge strikes the frizzen squarely.
If it seems too long in the jaws, you can notch the leather where it contacts the jaw screw and get a very slight reduction in how far it sticks out of the jaw (by the thickness of the leather).
I have also found one that is too short will either not spark enough and/or will not kick the frizzen back to fully open the exposure to the pan powder. Easy to adjust it forward should that happen.
And one last word. Be careful with them! They generally as sharp as a knife and will cut you badly. And when closing the frizzen keep your fingers out from the space between the flint and frizzen as it closes. Accidentally, I failed to do that two days ago and have a very nasty cut on my right thumb just above the nail.
More than you asked but that's my take.