I know there are members on here who have had no problem with their cats and rabbits getting along. Perhaps they will see this post and say how they introduced them. Some even snuggle together.
If you have an enclosed cage, that should be good to keep bunny safe. It depends on how persistent a cat may be in reaching through the openings of the cage and whether bunny is smart enough to back away if needed. I have no experience with this myself.
As for your cube grid cage, I would suggest making it no more than 2 grids from front to back. The width can be as wide as you like. The reason I'd suggest no deeper (front to back) than 2 grids is because if you go deeper, it will be difficult or impossible to reach all the way back for cleaning. Your arm will only reach so far!!
If you intend to have an upper level, you may want to consider having it go 3 grids high though. This allows for more areas of overhead space. It's good for a good portion of the cage to have 20-24" of overhead room so bunny can fully stretch up on hind legs without bumping the ceiling. --just some thoughts while you plan.