OK I'll break it down see if this helps.
In the Media Folder:
Classes>Models <object name.xml>
<Body Collisions="Dynamic" Friction="0.8"/>
Meshes>Models <object name>.X and <object name.xml>
Does that make sense?
Take the .x object (the physical model you made in .x format) and place THAT in meshes>models inside the Media folder. Does your model have a collision? "cdt" ? Copy the object you made in the Editor and name it cdt then save, and highlight the two selections, and select "export both" or the equivalent of what program you use.
I've yet to do it to my truck, but I believe on the right hand side you can choose the unit of measurement.
https://knowledge.autodesk.com/support/3ds-max/learn-explore/caas/CloudHelp/cloudhelp/2015/ENU/3DSMax/files/GUID-22D2D2B9-9A82-47FA-88DD-67B1A91E2337-htm.html This may explain better, as well as the Editor Kingpinn mentioned.