Sizing help -Medical research purposes- New member-first timer

Good morning

We are stuck at sizing. I´m working in zbrush but I do need this 3D print to be a very accurate and exact size since it is for medical research purposes.

I exported a obj. file from Osirix (from a DICOM file) to MeshLab where I deleted some unwanted artifacts and parts. Then exported a new obj. file to zbrush were I did the modeling. Once the work is done, I try to prepare the file using the 3D printer option in zbrush but the size it shows there is too big (mm) for its real life size purpose. So I haven´t uploaded the model because I need to get the exact size to be printed.

In the picture you can see the skull taken from DICOM file with Osirix. As notice I worked in zbrush where I modeled a part on the side. I imported the file from Osirix into zbrush to make it fit and it does as shown in the image. But the thing is how do I print this part (not the whole skull, only the modeled part) in real life sizes so it can fit the real scanned skull?

Please could you help me out I don´t know if I have been clear enough.

Thanks in advance

Open your file in http://www.netfabb.com/basic.php
There you can measure, scale and repair if necessary.

Determine the unit size that zbrush exports as a STL if you do not want a colored material.
Import into netfabb which will give you the bounding box dimensions. If you scanned a cube whose dimensions were 12" x12" x12" for example, after importing into nettfabb, the bounding box dimensions were 12mm x 12mm x 12mm, you would then scale the object by a factor of 25.4 to get it to print to the correct size, then export the new object as a STL, upload to Shapeways.
When ever you have a problem always make sure to put a link to a file you need help with. That helps loads. What might be driving you crazy, many here can fix after a few minutes, and get you set up. PM me for any help.