Hi,
here is a Googlized Version of my Openoffice-Spreadsheet (http://spreadsheets.google.com/ccc?key=0ApcctbKrMR0edHgyMEQxYk96M3ZKamVqc0NUS2tleFE&hl=de).
It is meant for converting Object Size Values between Pixels in Images and Points in 3D-Models.
There is also a small help for Positioning the Camera.
All these Calculations were meant for Blender, but they should work for any other 3D-Software as well.
Well, I hope someone will find it usefull. I can only hope that it's free of errors, there is no Guarantee.