A cage the size of a 10 gallon fish tank would be a good size for a pair of buttons, though fish tanks aren't very good since they don't allow for air circulation. 
Mine are in a rat manor cage, divided in two, so half the birds are on top, and half are on the bottom. They are not happy. The ones on top don't like being in the air, and the cages are too crowded. 
I plan on making cages out of a bookshelf, with each 'cage' being 2'x1'x1'. There will be 2 cages per 'shelf', with a 3" lip around the bottoms of each cage, to help keep bedding in, and in case of babies 
		
		
	
	
 I have 14 buttons, 5 females and 9 males, so I'll need a few cages, since the plan is to have enough females for the males I already have. So my bookshelf cage will need to be 7' high by 4' wide, and 1' deep. Both sides of the 'shelf' will be hinged and divided, to allow for easier access. 
Some people use rubbermaid totes, I haven't tried this, but I would think they would escape pretty easily, since the top would need to be opened for cleaning. 
As for their food, they really should have something high in protein, like game bird feed. They can have regular chick starter, but you would need to supplement them with protein, either from eggs or from cat food or similar. 
They are really neat birds, you would enjoy them