I'm going to buy 2 kind of rabbits. A Californian and a New Zealand. Can they fit in a 48x30x18 if so can I go with a 36x30x18?

this is definitely a lot of questions and very little information.

Are you breeding for meat? If so, I would chose one breed or the other, not both. Both are very good meat rabbits and will provide you with a great meat to bone ratio. If you don;t know what that is you need to do more research. Mixing the breeds is not recommended as you never know what your going to get and could end up with either birthing and kit problems or baby rabbits that can't be salvaged for good meat.

Obviously the bigger the cage the better. A rabbit should be able to hop across its cage, an stand on its hind legs without its ears touching the top. You will need a gave much taller than 18" for either of the breeds mentioned above. A californian rabbit will be a bit smaller and could be ok in a smaller cage, provided they are given ample time to run around outside their cage. If they are getting plenty of time outside their cages, the ones you have above should be fine for all of the questions. However go bigger if the rabbits will not be allowed out daily.

A new zealand white rabbit, however, will give you more meat. but it does need more room and more exercise time.

I suggest going onto an agriculture group on Facebook. I was able to find a great meat rabbit group for my specific state where people posted ads of the rabbits and supplies they were selling. It's also a great way to get your questions answered. Especially if your looking for alternate ways to kill or skin the rabbits. They have quite a few tips on preserving the pelts and meat

We used to keep both NZ and Californian rabbits, and each had a cage about 40x18x18. We never put two in one cage after they were adult, except if we wished them to mate, when we put the female into the male's cage for a short time. Two adult males - or females - in one cage would tend to fight. The size of cage above was fine for the breeding mums to give birth and raise her babies, but they need part of the cage to be screened so she has a 'private' area to give birth. The little ones should be moved to another cage as soon as they are weaned, and don't forget they can breed themselves within a very short time, so would need to be checked for sex and separated pretty soon. All our cages were the same size.