I use 36" x 28" for each adult corn.
It's personal choice how much room to give an animal.
The more room you give them to move, the more exercise they get which I suspect is good for them in terms of muscle structure, not becoming obese and is less likely to cause females problems during egg laying season.
Many people use plastic storage tubs, usually in rack systems, so they can tell you what sizes they find work best for them.
Skye