I really look at the jig skirt to adress three variables:
1. Action
2. Color
3. Size
I left out fall rate because that is something best done by switching to a different jig weight. Trying to determine the fall rate of every jig in my boxes based on the skirt material, length, diameter, etc is nearly impossible and cumbersome. Yes, a larger skirted jig will fall slower than a thin skirted jig, but sink rate is most accurately adjusted by swapping to a different jig weight. If I want a slower or faster fall rate, I swap jigs, not skirts.
Action: The amount of action a jig gives can be largely affected by the skirt. Silicon vs hair, round vs flat. If I want a lot of action I prefer silicon, but in colder months I will use hair to minimize the action.
Color: The color of the skirt is something that I take a close look at and try to mimick the substrate of the surface I'll be fishing instead of focusing only on the color of the craw. For instance, if I am throwing jigs on a sandy bottom, I'll make sure the the skirt has strands that are as close to the color of the sand as possible. I feel that the sand colored strands moving around while the jig is at rest looks like the creature is trying to dig around in the dirt to hide or make a stand, much like a crawfish might. Also, if I know i am fishing a jig in sporadic grass, I'll have multiple colors of green strands in the trailer to make it look like the creature is moving the grass around him in his defense pose when I have the jig sitting on the bottom. Its something I put a lotof emphasis on but I dont know if it helps much, more of a confidence thing if anything at all I suppose.
Size: Skirts obviously can affect the profile size of the lure. Longer, round strands with hair offer a thicker/larger profile (Hippy Jig), while flat strands of a lower count offer a thinner profile. Simply match what size bait you believe the fish to be snacking on.