When I bought my Turner Saddlery slings, the nice lady on the other end of the phone suggested a 48" for shooting an AR15 in NM configuration and a 52" for the M14. That's all I can remember. I'm 5'6" and not that tall with shorter arms (medium jacket) and that 52" works for me.
I selected the biothane version since I like the modern high tech look/feel/ adjustability. My sling is in TAN, which allows me to write (Black Sharpie) my settings on the sling, nearest the holes.
For those who have attended my clinics, I advocate 2 settings: One for 'show' and one for 'wear'. The show setting is nice and tight for our PPCLI Depot Friday morning inspections.
The "wear" setting is for American Hunter rifle sling carry (strong hand shoulder) which is long / adjusted long enough for a hasty sling.
Remember all the while, the loop in the forward strap is already adjusted for prone position "match" shooting. Lots and lots of vids on youTube to achieve this adjustment!!
Hope all this clears things up. I realize that this legendary sling can be overly complicated but once it's setup, you are good to go for LIFE!
Cheers all,
Barney