Target size (metres) = s
Target distance (metres) = d
Observed angle of target (mil) = a
Calculating range: d = s*1000/a
From which follows: s = d*a/1000
Picture illustrating observed angle of a 1mx1m object at 250m (observed angle should be 4mil by 4mil): http://cloud-4.steamusercontent.com/ugc/38620363499573714/D9F0FC29207FD8AB329B664F70BD1D06AC4EC043/
Video illustrating behaviour when calculating the range of standing soldiers: http://www.hitbox.tv/video/485652
The behaviour is consistent across all scopes I tested.