Can these full scene compser views be exported as single spritesheet?
Aslo, is the 128 limit centrally built into the architecture of the program or is it something that is somewhat easy to change?
Yes you can export the whole scene as a spritesheet.
128 limit is due to performance. I could probably work around this to support even bigger sizes but it's not the focus of this software - it stops to look like "pixelart" at that level of details and you should consider more effective technique if you need voxel objects of that size - probably low poly 3D would be a better fit. SpriteStack is for making game objects like units, chracters, buildings that look like games from 90's