If you have the time and effort you can make playpens as big as you'd like without worrying about bar space all you need is cardboard boxes and some masking tape, plenty of tutorials on Youtube.
If not pets at home have a decent sized playpen and the bars are the same width as you'd find on any standard cage. They can be pricey and you can sometimes find the same ones but cheaper on amazon I would take note of the pen size from pets at home as it is perfect for dwarfs and robos.
http://www.petsathome.com/shop/en/pe...nimal-play-pen