The one that I made was, I believe, a square of 30cm by 30cm, but had longer pieces at the corners (its an irregular shape). My single adult male can fit inside comfortably.
I typically make mine about 13-14" square, though that size really is meant to fit only one chinchilla without squishing. If you want it to fit more, make it bigger...
Oh nice to both ideas! I made a smaller one that hugs him and he seems to like the fit, but I wanna go with something bigger. I'm sure Midnight would absolutely love to be hidden in a hammock!!