They will always want out... lol, but the more room they have, the happier they will be. I suggest building it as big as you can, at least 8 feet long and just as wide if possible. A 10'x10' dog kennel (with a layer of chicken wire and or hardware cloth all around it for added predator security, and a top on it) would work great too. They'd also enjoy outside perches at different heights and a mirror that they can admire and amuse them selves with.