Did you notice new versions of Mathematica give volumes of shapes, and that if you save an Image[] of the shape, Mathematica can give you Area/volute measurements as well?

Did you try Volume Rendering (new feature in Mathematica)? It with "3D" render using {x,y,z} "cubes", which may be easier for you to work with "as a grid".

Mathematica will not distort Graphics3D if you choose your BoxRatios / Aspect ratios.

Mathematica has an option Screen Environment (see Format in main frame/top menu bar): "Working" "Printout". Also you can choose to "show ruler". Printout should show an ACCURATE representation (and if your OS is setup right, you can put a ruler on the screen to the ruler on Mathematica's ruler). Furthermore when you print if the inches/millimeter shown on ruler do not print EXACTLY as seen on screen (ie, 4" on ruler on screen is 4" exact on paper): then you have some OS / Printer problems or perhaps a Mathematica bug.

HOWEVER: Mathematica's GL I don't think is as accurate as the older version use of Postscript graphics as far as "measuring in em's 1/72 of an inch. Infact I believe you may find that you can find glitches in some GL renderings (i have a list of ones i found). Therefore make sure your 3D is "upright and proper" before using measurements for anything other than general study. I would have to print a page with Text (using fixed fonts known measurements in em's) and with 2d pictures and 3d graphs to see/insure if GL is rendering them faithfully to scale (given printer settings, etc). I'm not sure how Mathematica deals with 3D that does not fit onto page when "aspect" is 1:1 nor how it converts between Scaled[] (0-1 range) to em's - and note you need to Scale[] as "everything is relative" when your talking about 3D and giving range 0-1 let's you fix the relativity to em's, I'm only sure that Postscript graphics are/were faithful as are Image[].