at a minimum 1 square foot per bird. but for happy birds the bigger the space the happier they will be. Just please keep them in single pairs. I have my pair in a bird cage that is 18 deep 36 wide and 18 tall, its a finch flight cage. I cover the bottom wire with non skid shelf liner, Walmart has for less than $5, and aspen wood shavings. I also have fake plant leaves woven into the top so they think it is something over their heads and they do not boink like some have trouble with. They do jump up and flap their wings once in a while to exercise but have never bonked their heads. I also have a small box they can "nest" in and want to add a live plant they can hide in this summer. Be sure to give them oyster shell and grit and mine love the sand bath.
If you have an other questions please ask before getting your birds and good luck they are addicting little guys!