If you want meat and eggs, coturnix might be a better choice. Buttons are cute, but when fully grown they are about the size of my (female) fist - really not much meat on them. And the eggs are tiny as well, they lay lots of them, but you'd need many to get an amount similar to a chicken egg.
Pen size for buttons, the bigger the better, but at least 1 square ft pr. bird. I suggest not using a wire bottom for buttons. If the pen is less than 1½ meters high, the top should be soft material, to prevent boinking injuries. Some would probably suggest making the cage so low they won't fly, but personally I think they should be allowed to use their wings a little, so my cages are at least 50 cm high. If you are set on buttons, I can post a few pictures of my pens.