18" or 24"??

[MamaSheepdog]

Mine are 18" also. My Rex stand upright in them, usually when I am handing out yummy treats like horse cookies.

So, even when they're upright, 18" is enough?

I'm still trying to determine whether 30"x30"x18" would be big enough for a Cal doe with a dozen kits.

I think 18" is tall enough. A 30" x 30" cage would be doable, but I think you will be happier if you make them larger. My Rex does are in 24" x 30" and raise 6-9 kits, and they get crowded, which is why I built grow out pens. I wish I had them in at least 30" x 30", and I'd be happier with 30" x 36".

All of my rabbits get bunnybuckets, which gives them a bit more usable space, as well as a bit of exercise.

Here is Feather with a litter of kits, and a litter jug which I provide to the kits as shelter once the nestbox is removed: