I don’t believe there is a one-size-fits-all answer to this question. It depends on the bunny. In my experience, my bunnies are not happy in less than a 4x8ft space. Obviously they don’t make cages that big. So I don’t cage my bunnies. They have a bunny room. Guinevere never leaves the room. It doesn’t even have to be a cage. You could use a puppy play pen, which makes a 4×4 square. Or you could use NIC cubes and build your own habitat, and expand it as you find the space. I am aware of the “standard” size bunny cage that BanditCamp described, and while I know there are many people with happy bunnies in small cages – I can’t bring myself to put my bunnies in such a small cage. There are others who share my feelings, like Sarah. It all depends on the bunny. I personally like the british system – they recommend 4×8, but particularly, they say the bunny needs to be able to stand up and stretch up, so 24″ high also depends on the length of the bunny. For the length of the cage – the bunny should be able to take 8 uninterrupted free hops. That’s a pretty large cage. There are so many different pieces of advice on the most appropriate size. My very first bunny cage was a 2ftx2ftx2ft cube. Morgana chewed through it to get out. She also tried to chew through a 4ftx4ftx4ft cube. Now that she has a 6ftx11ft area (half the bunny room), she is happy. So if I were you, I’d scour the thrift store, craigs list (depending on the country you are in), look for second hand inexpensive cages/puppy play pens etc, and use that first cage as a test cage. See if your bunny is happy, before spending money on a more permanent/upgraded cage.
For your second bunny – a cage the same size, I’d personally put them next to each other with at least a 6 inch space so they can get to know eachother without being able to hurt each other through the cage.
For how long before putting the together – it depends on how long it takes to bond them. I’v bonded two different pairs in 2 weeks. My third pair took 8-10 months. It’s a good idea to post in the bonding area of the forum when you are ready to start that. Post a journal, so people can comment and help you along the way. I found it extremely helpful with my 8-month bonding experience – as I had expected it to go as smoothly as the first 2 pairs, and it didn’t. Before you start bonding, do some pre-bonding. Every day, swap cages, so they learn that they share territory. Once they stop the territorial pooping in eachother’s cage, you’ll know it’s time to start bonding. This can take weeks to months. Then there is the bonding – bunny dates, stress bonding (or not), allowing them to sort out the squabbles, preventing fights, there is a lot to it.
Take a look at the habitat forum – there are a number of pictures and ideas for cages/habitats/setups.