Giant bunnies need giant cages. Withthe number of rabbits that you already have in a small space, I don'tknow if a giant breed would be a good idea. The smallest I'drecommend is an XXL dog crate. I think they're 48 inches longor so. Bigger is better. Many large breed and giantrabbits are kept in dog exercise pens or in huge cages made out of wireshelving grids (NIC cages). Other people simply bunny-proofan entire small room and let the bunny have the whole thing.Smaller cages like the dog crates are okay, but the rabbit needs to beout for exercise a lot (at least several hours a day) to make up for it.
In addition to the cage, the bunny would need space outside of it torun and play. Although they have a reputation for being lazy,giant rabbits do giant binkies! They need enough space to runto get up to top speed and do their crazy binkies without smacking intoa furniture or a wall at every turn. With all your otherbunnies, do you have much floor space left in your apartment?
Also, are your other bunnies spayed/neutered? Are any of thembonded? Since rabbits can mate through cage walls, accidentallitters can be a real problem. And if the flemish is a boyand impregnates one of your smaller girls, she could be at risk forhaving kits that are too large to give birth to without surgery.
Sorry if I'm being a bit of a downer, but a huge rabbit in a smallapartment, especially with five other rabbits, is a very bigresponsibility. If you have room for a massive cage and stillhave room for the rabbit to play in, as well as time for all of yourrabbits to get exercise and attention, then a giant bun might be apossibility for you. I live in a small three bedroom housewith two pairs of rabbits (three dwarfs, one dutch) and two kittens andI don't see how I could squeeze a flemish giant in here without feelinglike I was neglecting him.