Correct! The next one is two inch, then shortly after that, it gets bloody big! If you where to build the 'half' Imperial, what driver/s where you intending to - err - drive.

The 12" Electrovoice Wolverine speakers should be great, in which case you may end up building an Imperial about 2 foot wide.
Or, you could build two 1/2 Imperials with an 8" + 12" in each cabinet!
You've got me thinking! I've got a pair of twelves (or is it tens) still in the boxes, hmmm, maybe I could try 2 x 8" +  12" + the Vifa tweeter. As I described in my build, the baffle plate (what the speakers are fixed to), is removable and interchangeable.
If you are concerned about using different sized speakers in the same Imperial, don't be. Check out some of the earlier posts, you'll see some wildly differing differing drivers in the same box.
The design of the Imperial almost makes this mandatory and of course, the original texts where based around this delta.
Me thinks I might be buying a piece of board next week, time I had another play.
Gaz

Thank you for the encouragement JimP