Simple question about measurement

I know where to in the 2.66 UI to find the scale measurement of my object which shows the size of each individual face, vertex or edges and I know how to show metric or imperial measurement. What I really need to know is how so I find the measurement of the length, height or width of my object as a whole rather than individual faces? I’m trying to work out the settings for subsurf scattering and to work out the scale of SSS I need to know the size of my object as a whole and divide the blender units by real millimeters.

How do I work this out?

Anyone? I think it’s a really stupid question but honestly I don’t know how to measure my model and see what size it is in blender units. It can’t be that every model I make is exactly 1 blender unit right? Because that doesn’t make any sense :S

In edit mode there is a mesh display section in the properties panel (N) from where you can turn on different edge and face info.
Development version of Blender also comes with 3D print toolbox addon and ruler & protractor tool.