TLDR: select your tank model directly in 3D viewport window.
There is the cue. I find that the selected icon in your outliner window is an empty object. And the tank model is not highlighted. So, actually what you've selected is an empty object. And the properties showing up now is all about the empty object, not your tank model. An empty object doesn't have a "dimension" property in that panel. I guess that your tank model is a child of this empty object. We usually make an empty object (like an axis) as our model's "root".
Solution: make sure that you are selecting a REAL model which will be highlighted.
In blender, this properties panel only shows the information related to the selected object. And here is the thing, selecting a parent object doesn't means that you're selecting the whole group of objects.