Unfortunately there's no general rule about sizes and measurements in Poser. You can find out more details by Googling "Poser Units".
However, I had to choose one, so I picked the one which appeared most logical to me.
Using the metric system like most of the civilized world ;P I chose that when setting "metres" as the reference scale, a translation of 1.000 logically equals one metre. This became my standard for all scenes. SASHA's body height is calculated by this reference too.
(to your amazement you'll notice that "V4.2" by default is actually 192 centimetres tall!!!)
First you'll have to find out how tall a girl of the prospected age might be.
To give you a head start I've already compiled a list of *average* body heights and built that into a dial:
The "-Preset Height By Age" dial.
You'll find it in the BODY actor, "TRUE AGE & HEIGHT" in the parameters list, below the "General" group.
Just dial in the age and the body will be scaled to the approximate height.
(Please set this dial to 12 for this demonstration!)
Now have a look at the two other dials underneath:
How to read these values?
And then? Two choices:
In the latter case, first reset the "Preset Height By Age" dial to 100.00.
Next you set the "-Approximate Height in Centimetres" to the value you wish (also useful when creating adults).
Read the actual height from the "-True Height in Centimetres" dial.
Adjust the "-Approximate Height in Centimetres" dial until the "-True Height in Centimetres" dial shows the intended height.
NOTE 1: Only the "-True Height in Centimetres:" dial shows you the true height of your figure!
NOTE 2: You can't set the "-True Height in Centimetres:" dial directly. This is due to a limit in Poser's ability to "calculate" with parameter values.
Now you have a figure that's exactly to scale.
Unfortunately most of the scenes and props aren't. As a rule of thumb, most are around ten to twenty per cent oversized because of a misconception of scales in the past.
So you should fix this to get believable scenes.
Of course you could just load your scene or prop, scale it by rule of thumb, and use it. This can work.
But don't underestimate the "eye's measure"!
If the door is too low or the table/chair/bed too high, our eye measure notices this and puts up a "this is looking wrong" flag. Mission failed..
So to get really exact results I'd suggest a little more challenging, but much more exact procedure.
You'll find this tool in the "Karina's Tool Box".
When you load it into your scene it will be at xyz 0.0.0. and exactly one metre tall. A more detailed description on how it works can be found here.
Use this tool to measure and scale your scene to the correct values. Some standard measures can be read (but not set! Because the tool can't know what you're actually measuring!). They serve as an ample guide.
Just place the tool near to what you want to measure, set the intended height/length/width and scale your object until it fits.
If you have a scene with several props it makes good sense to create a "Grouping" from Poser's Menu "→Object →Create Grouping", and then parent the whole scene to this grouping.
Now you only have to adjust the scale in the "Grouping" instead of everything individually.
DON'T FORGET to save your "scale corrected" scene (WITH the "Grouping") to the library for further use!!