Here are shots from two of the shelves from my collection, I wanted to put together a rough scale set on each shelf which compared the craft to each other, this is the best I could get. The scales are only approximate, but all around 1/320th scale, with the larger TB2 around 1/144th.
I put this together some time before I saw Chris Thompsons beautiful chart of comparative sizes, but curiously enough they seem to roughly match. That being the case, in would indicate that the Zero X here would be in the region of 700 foot, or twice the size of a Saturn Five.
Of course the difficult thing is, Century 21 made no real effort to build any of the craft to scale with each other! So the sizes can vary considerably.
I always likened the TB2 Pod to a magicians hat, as sometimes more stuff would come out than it was possible to carry! Having said that, in the International Rescue Owners Manual, there is one Pod configuration where the Elevator Cars are stacked on top of each other...
Hope these photos help, as they do stack up in comparison to each other, and Chris' chart too.