How can I trim an object by an exact mesure? Is it possible to trim an object by 1mm from each side?

Scaling isn't really an option, because I don't exactly know the factor. I just want the model to be 1mm narrower from each side (each vertex should be "pushed" inside by 1mm).
You can modify the size of the object with the x, y, and z size spinners the toolbar on the right of the 3d view.  All input fields can accept simple mathematical expressions so if the current size is .5m you can type “.5-.002” (.002 not .001 because you want 1mm off each side) and it will scale the object accordingly.