Flemish: cages and rabbit proofing
A cage for a Flemish Giant should be 48" x 48" x at least 27". It can be a bit smaller if he spends a lot of time roaming the house. I use a dog crate. Mine spends a lot of time outside the cage, under my supervision. A minimum of 2 hours in the morning and 2 hours in the evening or more. He sleeps in his "room" (the crate.) He is not left alone outside the crate just yet because he is still learning the litter box. He also has a harness and leash for going outside about 3x per week, which he loves. Organic grass only! Don't let him graze in fertilizers or pesticides. Plant a small organic patch of bunny safe plants for him to graze in if you can, but no tomatoes. The leaves will make him sick although he can eat the tomato itself.
Don't worry about him getting lonely. They will make friends with just about any other pet you have. My cats took to him right away (with careful introduction and a little patience)
Rabbit proof everything waist high, maybe even higher for the Flemish. This means heavy plastic wrapped on wooden table legs, heavy plastic tubing on all electric cords, basically heavy plastic on anything unsafe to chew. Rugs and carpets will be chewed and ingested. Enough of this and it will be an expensive vet trip for intestinal blockage. This is another reason your bunny should not be left out unattended.
All in all, Flemish are high maintenance, so be prepared to spend a lot of time and money caring for him/her. Anyone considering a bunny should know this because the shelters are absolutely overflowing and I think this is why.