I wasn't sure what I length I would need when I got mine, so I just took a chance. There was a guy with M14 slings and parts at his table at the last fun show. I figured if it will fit on an M14 it will fit on my NMA4 AR15. He didn't know how long they were, he said that he just ordered M14 slings from Springfield Armory.
I just went and took some rough measurements of my sling.
The long piece is about 47" and the short piece is about 24". The way I have it slung up now, it is just about right (or at least it seems to be long enough). I have about two or three holes left to adjust longer if I need to and a whole lot of holes to go shorter.
Since it seems to matter, I can tell you that I am 5'10" and use the same setting for sitting and prone. I also wear a sweatshirt and light weight shooting jacket when I shoot.
I went to the Turner Saddlery site and clicked on the NM sling. It says this:
National Match Service Rifle Sling Competition Sling made from 11oz strap leather, with numbered adjustment holes and a 50" strap for those who find the standard 48" length too short. Width is 1 1/4". Fashioned after the M1907 sling. Fitted with MilSpec Parkerized steel hardware and brass rivets. Short strap is stitched with bonded nylon thread to resist rot. Keepers are 7/8" wide and hand stitched. Longer sling lengths available at no extra cost! Sling comes with one pair of heavy grade competition keepers. Swivels will need to be purchased separately.
http://prostores3.megawebservers.com/turnersling_com/-strse-10/National-Match-Service-Rifle/Detail.bok
Based on that I probably have the "standard" 48" sling. If you buy the 50" sling you should have at least three or four holes left for adjustment (assuming you are average height and using average shooting positions.)
EDIT: I just read your post that says you are a giant. I would recommend going with the 56".........For what it's worth, the leather sling will go through the front swivel, down to the other end, then hook onto itself. If you try to have someone hold a fabric tape measure around your arm and gun while you are in position, I would have them hold the 1 on the front of your bicep, wrap around your arm, through the gun swivel, go back to the beginning of the tape, and then add 4 or 5 inches for the loop back. That will tell you how long a sling you need.