1st Section

 15mm

Rather than being a scale 15mm (or 20/28mm for that matter) is a measurement of a figures height. In the past this seems to have been the height between the ground and the top of the head of a standing figure, but now is typical measured to the figures eyes. Regardless of this figures described as 15mm are likely to vary in height by a couple of millimeters. This will have no effect on game play but may affect the aesthetic on the table if noticeable. 

Peter Pig are my 15mm figures of choice. They are metal rather than plastic or resin and so have that nice heft and are also robust. The level of detail sculpted into the figures is impressive and not always noticeable on their website, and their WW2 range is extensive, particularly for British and German troops.

Not perfect measurements but enough to get a good feel for the size of their late war figures. (The earlier sculpts seem to be a little smaller)   

                     

     

15mm is generally considered to be 1:100 scale. Simple maths confirms this. A standing man that is 5'11" (1.8m) tall will be 18mm high - very close to the figure above.

Model vehicles and buildings are typically quoted at their scale, and for 1:100 the maths is very easy if working with metric numbers - just divide actual size by 100 - very handy when printing STL files to ensure they are the correct size.

Chain of Command

The quoted ground scale for CoC is 12" equals 40 yards (1"=10ft). That is a scale of 1:120 and so given the variance of 15mm figure heights is pretty close to matching the 15mm ground scale. Interestingly when scaling a 6'x4' table we get a battle area 220m x 146m (240 x 160 yards) which is a surprisingly small area. This shrinks to 102m x 68m (112 x 75 yards) if 28mm figures are being used which are generally considered to be at 1:56 scale. Therefore there is a degree of abstraction creeping to the game at that scale, whereas  when playing with 15mm what you see is what you get in terms of weapon ranges and the like.