ac3d will not let you identify a space. You can label an object such as a box so you could create a box inside your bookcase "cell". The box would have the same dimensions as your "cell".
ac3d keeps track of where vertices, surfaces, and objects are, but it has no way of knowing what those make when they are put together like a box. Consequently, it would have no way to calculate the height, width, and depth of an object. You can create an object called a box, but ac3d doesn't actually recognize the object as a three dimensional kind of thing with height, width, or depth dimensions. It only keeps track of the ** locations ** of the vertices, surfaces, and objects.
To do what you want, you will need to write your own program that will read an ac3d file, look for the box-like structure that you labeled as "your_label", read in the vertex coordinates, and then calculate the height, width, and depth as you desire. A useful resource is the ac3d file format which can be found at
http://www.inivis.com/ac3d/man/ac3dfileformat.htlm. This document will show you how ac3d keeps track of everything and should give you enough information to allow you to write your program. ac3d keeps all of its data in a text file with the extension ".ac". You can read an ac3d file in Wordpad or other text editing program.