Using this approach the cube's floor face would mirror the texture size (note that I would think using 3d this way would make using the 2d diamond shape tiles I assume (based on the dimensions) you have complicated).
That's exactly the case -- up to now. Thank god I haven't drawn hundreds of assets, yet. ;) So you're right, the images contain diamond shaped graphics.
Hope that helps and isn't too confusing. I often have a tough time explaining such concepts.
Wasn't confusing, but helpful. The only problem I see is what you mentioned above, namely box sizes to match the texture. If it's possible (and more or less easy to handle) I'd keep the diamond shaped tiles. Otherwise it'll be hard to paint walls, for example. I would have to use real 3D geometry for them.
Maybe it's more helpful for the reader to see what's in my mind:
It's a screenshot of my current engine with software z-ordering and I'd like to keep the graphics -- or at least the geometry of them.