Imagine a scene containing multiple objects, for example a bed and a chair. The bed and the chair aren't blender-objects themselves, but they consist of many subobjects. To view the dimensions, rotation etc of one of these subobjects is not a problem, but what if I want to see the dimension of the whole bed? I have thought about parenting all sub-bed-objects to one of the parts or adding them all to a group. However, so far I have not been able to figure out how to display the dimensions of a whole group. How could I do that?
I have created an addon that will create a mesh cube that encompasses all selected objects. Of note is that non-render objects such as camera, lamp and lattice objects report there bounding box as a zero size cube, this can lead to a cube of zero size that is only seen in the outliner.
To use you can save this link to your addon folder or to somewhere convenient and use the Install from file button in the preferences. You will then find Create Bounding Box under the Add Mesh menu.
This will give you a single object that clearly shows the outer dimensions in the object properties as if all objects had been joined.
Blender doesn't support showing dimentsions of dupli-group instances or children (directly), You can however use the Ruler to measure it, hold Ctrl to snap to the extents of the objects.