How much space do chickens need if you want the gr to live?

Does anyone know what the area of a run should be per chicken if you want to keep the gr green? On my farm free ranging is not an option, because of our garden, and because there are a lot of feral chickens, and I don't want the wild roosters harring or abducting my hens. I figure the next best thing is to fence in an area large enough that it won't be reduced to dirt. Maybe one hundred square feet per bird? Five hundred? We have considerable rainfall, and tropical sunshine, so the gr growing conditions are good. Thanks for any info, anyone.