2’ x 4’ should be plenty big enough if you move her. If you put her on the ground the nest does not need to be real big, 12” x 12” will be plenty big enough. Then provide food and water. She should come off the nest once or twice a day for food and water and to go poop. She should not poop in her nest but you may have to change out the food or water kind of regular. They know not to poop in the nest and mess the eggs up but they don’t know to miss the food and water.
You will need to lock her in that enclosure. Do not give her the option to go back to her old nest because she will. Her instincts are on the nest, not the eggs. In a flock there could be several nests with eggs. She has to know which are hers and that’s tied to the nest.
Your big risk in moving her is that she may not accept the move. I suggest moving her at night using as little light and commotion as possible, after it is dark and she has settled down. Normally you do this and use fake eggs before you give her the real eggs to see if she will accept the move but you don’t have that option. You’ll have to use the real eggs.
When I move a broody like this, I lock the hen in the nest the next day, not allowing her to come off the nest at all, until just before dark, then I give her the option to eat and drink. Hens build up a fat pad before they start to lay, let alone go broody. This excess fat is for them to live on while they are broody and not eating or drinking much. They do need to eat and drink while they are broody but with that fat they don’t need to eat and drink that much. It won’t hurt her to stay on that nest all day after the move.
I suggest you make the nest fairly dark. Broodies like to think the nest is hidden and it being dark will help with that. Make your pen out of chicken wire, that’s fine, but totally enclose the nest and leave just a fairly small hole for her to use to get out and in.
Mine typically hatch in nests two to three feet off the ground. I’ve seen a broody hen get her chicks out of a ten feet high hay loft. Mama says jump and they do, then hop up and run to her. The jump or fall won’t hurt them. I really don’t worry about them falling a few feet. The problem may come in your nest design though. It’s pretty normal for the first chicks that hatch to climb up on Mama’s back while she is hatching the later eggs. If the nest is so small that Mama is sitting close to the edge, the chick might fall totally out of the nest when it falls off her back. I had that happen several times when I let one hen hatch in a nest made from a cat litter bucket. Not a cat litter bin but the bucket the litter comes in. She was just too close to the edge. Mine normally hatch in a 16” x 16” nest with a decent lip across the front. I’ve never had one fall out of those.
The chicks absorb the yolk before they hatch. That’s what they live off of while Mama is hatching the late comers. They can go three days or more without food or water because of them absorbing that yolk. That’s why they can be mailed, they don’t need to eat or drink for that long. It certainly won’t hurt them to eat and drink in those first few days but it’s not a priority for them. Mama typically keeps the chicks in the nest with her until the hatch is over, but don’t be surprised to see the chicks roaming a little before she brings them off. Each hatch is different. They don’t all always do the same thing.
Good luck with it. You are in for an exciting time.
