I second everything JaeG said and would like to add that you should feed them a game bird starer crumble with AT LEAST 19% protein in - and such a feed should already have vitamin D in it, so supplementing more of it shouldn't be required.
One thing a game bird starter lacks though, is calcium. Once the hens mature at around 6-8 weeks, you should give them a bowl of finely ground oyster shell or a different calcium supplement so they can take what they need to lay their eggs. This should be available at all times.
Other than that - my preferred cage size for a pair or trio of adult buttons is 4x2x2 ft. 2 ft high to allow them to fly a little - they seem to like stretching their wings in the morning. I pad the top of the cage to prevent injuries. 2 ft deep because I can't reach any further in than that. And 4 ft long because that gives me room to add tunnels, fake plants, blocks of wood, a sand bath, feeders and water while still allowing them plenty of room to move around and room to get away from me if I'm rummaging around in one end of the cage.
Many keep them in smaller cages and mostly that works as well, the ideal way to keep them is of course in aviaries, green houses and such.
Mine get broody and hatch their own chicks. While 2x4 ft might seem large for 2 birds, it's a different story when there are suddenly 10 ^^
As chicks they are prone to drowning or getting wet and cold, so make sure you have a shallow water dish and preferably add marbles, pebbles or similar to it so they can't get wet.
As adults they are VERY prone to getting their bedding into the water, causing it to become a nasty, smelling soup within hours - placing their water on a brick or similar helps. They can also learn to drink from rodent water bottles.