The best way is not by size - as chickens do come in different sizes. The best way to find out is by the comb on the head and the waddle under the chin. If you have a picture it would also help.

If your horse has colic you must keep it on its feet and moving. If you let it roll his intestines may knot up and you may lose him. Get a Vet ASAP.

It could be a blood sugar drop. You need to see a vet.