The 10" x 18" size seems to be standard for all but the dwarf or giant breeds.
I would suggest a cage with the wire in a vertical position 2" high x 1" wide. I attended a show yesterday, and as my friend was placing my Beveren buck back into his cage, he stuck his foot through the wire and could easily have broken his leg in his panic.
I have carriers made by both K-W cages and BASS equipment, and one other supplier which is the "problem carrier" mentioned.
The K-W have solid metal dividers, while the BASS has a 1" gap between holes. If your rabbits spray urine, you will want the type with metal dividers. I opted for the BASS carriers with plastic pans, and do not recommend that option. Go with metal pans as the plastic ones become brittle over time.
If you can transport the rabbits to a show in a dog or cat carrier bedded with pelleted pine or other absorbent material, you can buy the carriers at the show. Most and probably all vendors will take pre-orders if you decide beforehand which type you prefer, which will save you shipping charges.
However, pre-ordering just a couple cages is probably not necessary, since they always have a good stock of carriers at the shows I have attended.