depends on the slinger.
But generally longer slings are for distance and poor with accuracy and shorter slings are for less distance and better accuracy.

Most of us end up with an edc sling that compromises accuracy with distance.

I'm 5 foot 6 and my edc is 28 inches. I can use longer and shorter slings but prefer this length for most things.

What c_a said.  I like to experiment with different types and lengths of sling. I'm 6'4" with slightly disproportionately long arms.  I'm comfortable with slings from 24" to 60".  My edc is around 36".

Also, I don't have a lot of problems switching from one length to another.  I like a shorter one for the woods or skinny trails and a longer one if I know I'll be slinging in a more open area.  Most of us carry a couple of slings anyway, to cover all of the bases.  I've been at this for not quite a year now and my accuracy is probably about average, though it's slowly getting better.  Staff slings are way cool and once you get a feel for regular hand slings I suggest giving them a try.  A few of us have played with them and I've yet to read about anyone not liking them a lot.  The advantage to them is that you can hurl much heavier ammo.  

My 32" PJ works great with a helicopter style of slinging, lots of speed it built up for a powerful throw but for GP a 26" or so sling works better for me right now.

I've recently been toying with an overhead single rotation/sidearm release type throw that I'm getting fairly accurate with - My only gripe is that I don't have enough power. Perhaps I should try a longer sling? My standard length for virtually all of my slings is around 26", because I use my previous slings as a length reference when I make new ones.

the thing is, slinging is all about leverage.
essentially what we are doing is making our arms longer and removing the bones from the end section.

So if the second section (sling) is the same length as your arm, your body is already geared up to use it. The centre point is you hand and you're used to things the length of your own arm.
different lengths will require more mental adjustment.

distance is down to a lot of things but the main one is how fast the pouch is travelling at point of release. Now long slings are not as easy to move fast as small slings. But taller people with longer arms should be able to handle longer slings better and get them to move faster.

I say 'should'. because the record throw with a sling stone was by a relatively short guy with a disproportionately long sling.
so technique is actually more important than how long your arms are.

On the other hand the current record (with a captive release sling and dart) was by the same length sling as the stone throw but by a much taller man.

In short longer slings can be better for distance but technique is everything 

actually, it may just be because you are unused to it.