If they are in a permanent run - no matter what the size - they will eventually eat all the greenery.
I only skimmed the posts so I'm sorry for repeating if this was already suggested - but what you might want to try is setting up several 'grass boxes'. Make a 2x4 frame covered in hardware cloth, whatever size you want - but make sure that you are able to attach the wire TIGHT and with supports if you make it a big span. Use a rake to scratch up the ground under the frame - add a little compost or top soil or even potting soil. Then cover in seed and set the frame over it so that the wire is on top. This keeps the chickens from having access to the base of the grass.
As the grass grows up through the wire, the chickens can 'graze' it. You could set up several and have a different seed in each one if you like.
I only skimmed the posts so I'm sorry for repeating if this was already suggested - but what you might want to try is setting up several 'grass boxes'. Make a 2x4 frame covered in hardware cloth, whatever size you want - but make sure that you are able to attach the wire TIGHT and with supports if you make it a big span. Use a rake to scratch up the ground under the frame - add a little compost or top soil or even potting soil. Then cover in seed and set the frame over it so that the wire is on top. This keeps the chickens from having access to the base of the grass.
As the grass grows up through the wire, the chickens can 'graze' it. You could set up several and have a different seed in each one if you like.