First, I am not an expert. To some extent what I say comes from others, and to some extent it is the result of making mistakes.
Figure skating blades are actually measured from the back of the back mounting plate to the front of the front mounting plate, not by the full length.
Blade length and placement are complicated. In brief:
1. If the back of the tail is too far back, you keep accidentally stepping on it with the other skate, and trip. In addition, it is impossible to have the "neat feet" (which means close together) that figure skating judges tend to expect.
2. If the back of the tail is too far forwards, the corner at the back drags, and slows you down. In addition, you may tend to fall backwards. In addition, the amount you can roll from back to front without scraping the back or touching the toe pick, strongly affects how much speed and power you can generate. More length = more speed.
3. Jump landings could be awkward if the back of the tail is much too far off, in either direction.
4. The effect of the back of the tail can be affected by the forwards/backwards shim of the blade mount, because that affects how easy it is to reach the ends. E.g., if the blade mount includes extra stuff between the rear mounting plate and the heel, that will pull the tail more off the ice, but drive the toe pick closer to the ice. And vice-versa.
5. AgnesNitt has it right, IMO, on what she calls the rocker, and I call the sweet spot, which is where the longitudinal curvature (also called "rocker") changes, and represents a place where it is easy to turn and spin. It also represents a place along the blade that you can feel, which helps with subtle control issues. Though, I don't much like having the sweet spot in the least behind the ball - I feel like I over-balance it, and tend to roll all the way to the toe pick without meaning to. Right on, or a mm or two ahead is what I like - but this may be individual preference to some extent.
6. If the toe pick is too far back, and/or the back tooth is too long for your skill and experience, you tend to jam it into the ice, and maybe even trip, when you don't want to. At first, the toe pick will be nothing but a nuisance most of the time. As you become better, at least if you jump, you will want blade designs that bring the toe pick a little further back and a little closer to the ice, though preferences vary a lot.
7. If the toe pick is too far forwards, and/or the back tooth is too short, it is hard to roll to it, which you need to do for jump takeoffs and landings.
8. Some people, by the way, also use the toe pick during some turns and spins, especially at the start of scratch spins. So, you may want it to be easier or harder to reach the toe pick for those things too; as with other things, a farther back toe pick, and/or a longer back tooth, are easier to reach.
9. The effect of the toe pick can, like the tail, be affected by the forward/backward shim of the blade mount.
10. There is also side to side position. A little offset from center may help you balance, though it can also be handled by modifying insole shape.
11. There is a rotational alignment - you want it to be easy to skate forwards, so if you offset the back to one side more than the front, or vice-versa, you won't tend to skate in the right direction.
12. And there is a vertical alignment - when your leg is vertical, you probably want the blade to be vertical too, else you will have trouble getting good edges, and tend to skid. Side-to-side shimming can fix this.
13. The blade should be mounted in such a way that mounting does not twist or warp the blade in any way. The blade should be straight, not warped or twisted.
I'm sure an expert can add other things to that list.
Do you have a specific reason for asking?