As the LT said, there isn't much in terms of specific target dimensions. I'd say all Arma 3 character models are between 71-74" in height and approximately the same build.
Ranging targets anywhere from 400-1200m is as easy as getting the range with your Vectors, adjusting the zeroing of your optic or adjusting your aim with an ACOG, and taking a few spaced out shots to determine where you're hitting until you're on target. As far as anything outside of 600m, you really shouldn't even bother engaging.
As for being a scout/sniper, again as the LT said, the closest we have is a Designated Marksman. A DM holds the billet of a Rifleman in an infantry squad and only employs the DMR on the command of the Platoon HQ. Platoon HQ can select any of the 6 Riflemen in a Platoon to use the DMR, but usually select one and that Rifleman only uses the DMR when HQ deems it necessary. If HQ determines that a DMR is not necessary for the duration of the mission, then the Rifleman will fulfill his/her normal duties within the squad.