A square cage 18x18x18 inches is a suitable size for an individual budgie. There should be enough space for several perches and toys and still be space left over for the budgie to flap around easily. Bar spacing on any cage should be 1/2 inch or less. Anything wider is large enough for your budgie to push his/her head through or even get it stuck. This can lead to your budgie escaping, serious injury, or even death.