Hi Gaston, are you talking about in a particular view?

One problem with what you are asking for, is that in a perspective view there is not a 1 to 1 mapping between model units and the screen, because of perspective foreshortening. For example in a perspective view an object that is 1000 inches in width can actually be smaller on the screen than something that is only 1 inch in width, if the 1000 inch one is far off in the distance away from the eye point and the 1 inch one is close to the eye point.

So that's one issue - that there does not exist a constant model unit to image size conversion at all if you're referring to the 3D perspective view.

Are you trying to do something in the something like the Top view or in the 3D view with Projection = Parallel enabled? If so then I think it could be possible to come up with a solution for that. With an orthographic view there is a constant relationship between model units and displayed size, which is controlled by the viewport zoom factor, and it is possible to set the viewport zoom factor through script, here is a previous post relating to that:
http://moi3d.com/forum/index.php?webtag=MOI&msg=3913.1

It could be possible to retrieve the viewport zoom factor by script and copy it to the clipboard or something so that you could then calculate how many model units the image was spanning, but would you maybe like it better if your Ctrl+P script was modified to set the view zoom factor during the screenshot process to a set value so that you would always be producing screenshots at a specific model unit size?

- Michael