Just my opinion and they will vary 600 x 1.5 meters should be ok bit small for my liking but adequate, if you are letting them out or working them should be fine, I have never kept ferrets inside and never will ? I will say the bigger you can manage the better for them obviously and ferrets love company, keep in mind weather you intend to breed or not and do some research into what is required if keeping more than one. I would imagine it would be a pretty lonely existence for a ferret kept alone for a lifetime, ferrets like any pet are a commitment and not one that should be undertaken without