Figuring out what size for a dog crate enclosure

Recently, my friend got a second rabbit, but the new boy doesn't seem to like him at all, and he doesn't have to time for two, and the new boy has been a lone bun his whole life, and doesn't particular seem to like sharing a house with another. So, with this all in mind, my friend has made the decision to hand the bun over to me. The bun gets along with me, loves to follow me around the house, and has more room at my house.
We were told by the girl we got him from, that he's a flemish giant, but I don't think he is. He seems small for a one year old flemish giant. He's definitely a bigger rabbit. He's going to be a free range bun except for when I leave the house, so I just want to get him a dog crate. The cage he came with is way too small for him.
The attachments are him next to a 13-15 inch ukulele (I couldn't find anything else to set next to him, haha!)

I have an extra large dog crate that I think is a good size. I think that's the biggest you can get. The measurements are 48" long x 30" wide x 32" high. It has room to build a shelf if you want to. I'd really recommend getting one with two doors. Mine only has one door and it's hard to reach all the way to the back of it.

My buns are both in a 36" dog crate. they have lots of room to bop around and for their litter box/toys and water bottle. It was all I had when I decided to adopt my first rabbit, so when I got a second one, I wanted to treat them equally. My rabbits are young, (9/12 weeks) but only will get about 8 lbs

I feel like I need to go look at some, because I have a hard time visualizing how my boy will fit in each size.

My two buns (3 lbs and 5 lbs) share a 42 inch dog crate. I've added a shelf to part of it. If you have room for the 48 inch which is generally the biggest one, that's your best bet.

That's what I need to see. I live in a 4 bedroom house with 3 other girls, and I already have a giant critter nation rat cage in my room, haha. But I have a finished basement, but my roommate has a bedroom in the basement, and this bun will chew bars while in his smaller cage, not sure if he will in a larger cage. But I also have the "utility room" with the water heater and stuff, that's further away from her room with 2 walls in between.

I hear you! I'm a vet student living with 3 other vet students and there are 8 animals in our house.

I currently use a large dog crate that I got from the local animal shelters thrift shop. Definitely get one with 2 doors or it's really hard to reach to the back. I've crafted shelves in the crate for previous rabbits but the current pair have no idea they can jump so I havn't bothered.

If you have the room, a floor that's easily cleaned, and can find one cheap (they're really expensive new) you could also try an x-pen. I have one that I've taken a couple panels off of to make it a bit smaller. Depending on wether or not your rabbit is a jumper you might have to get a tall one (mine is 36 inches tall) or cover it with a sheet or some such. Once my new pair are potty trained, they'll be moving into the x-pen.