Dimensions of a duplicated group

Hi all,

I have an object A which duplicates a group of objects (which are again duplicates of even more groups). The objects inside of the group are animated and change size, so that the size of A changes as well.

My goal is to keep the size of A constant in the x-direction. For that, I thought of using a driver bound to the scale x channel with something like:

target_size / bpy.data.objects['A'].dimensions.x

However according to the Datablocks viewer its value is 0, just as all the corners of its Bounding Box. It seems that group duplication operator somehow blocks the propagation of the bounding boxes.

Can someone please give me a hint how to get the bounding box size to use in the driver, or any other method to achieve this?