I'm 6' 3" and I have long arms. I think my arm span is about 6'6".
My three slings have the following lenghths.
37"
42"
48"
I use the Jurek pocket design made with leather from Wal-Mart. All my slings have a leather "knot" and the cords are store-bought braided nylon. They are a little thicker than parachute line.
The 37" was built for throwing lead. It has a small pocket 1.5" X 3.5" I hardly ever use it because I'm not into throwing my money away.
The 42" I use for everthing except for tennis and baseballs. The pocket is 2" X 4".
The 48" is unique. It has two layers of leather in the pocket and is 3" X 6". I can "fan" out the two layers and make the pocket deaper and wider. I use this for throwing big stuff like baseballs and tennis balls.
I use a style that might be termed the figure 8. It is the same style that is shown in the sling video that cracked like a whip. My slings don't have the mass distribution for cracking, but I bet I'm shooting rocks at over 60 m/s. Perhaps 100 m/s on occasion. I like ammo that weighs an ounce or two. I am not that accurate. I can probably hit a refrigerator about 3/10 times at 100 feet. The other 7 times I'd be scaring the refrigerator real bad.
I really want to learn the pirouette style. There is an account in the Book of Mormon where a lone defender slays 6 attackers with a sling. Since I've become familiar with the pirouette style, I've imagined in my mind that the pirouette style was used in this instance. I believe that this style is best for throwing large ammo (3-10 ounces). Just my opinion.