Going back to calculating the size of the supercube...
I've noticed during a rewatch that during the supercube's destruction it seems to start to break up into lots of smaller cubes. Presumably therefore it's reasonable to assume that it's formed from a lot of regular cubes aggregated together. Since the sections it's breaking up into aren't all the same size I looked for the smallest cube size we see during the breakup and assumed this was the standard 3km Borg cube, with the larger units being groups of multiple cubes that were retaining some cohesion.
If we do this then it turns out the side length of the Borg supercube is...
approximately 47km. Well well well. What an interesting number...
If the supercube were made of multiple cubes docked together like building blocks it would explain why it has internal weapons. It would also explain why it has such large internal voids if regular cubes were missing or cannibalised.