A while back I had put a pair in an 18 by 18 by 30 cage with half an inch bar spacing when I was cleaning their cage. One little guy got his head caught in the bars at the top luckily I noticed and was able to squeeze the bars open. Goldbreasts are great little waxbills and Hearty too. The advice you have been given on bar spacing is accurate. If they don't escape they will get their head caught in 1/2 inch bar spacing. If they have babies they can easily fly out of that spacing. 3/8 is works well and mine have never escaped. As well I would consider a larger cage as they like to fly and will make use of every inch! Good luck!
