It depends on where you live and how cold it would get.
Here, in Wisconsin, we would hang that by a chain to a hook in the ceiling so we could lower it by a link or more when it gets colder.
I know of those that use those as brooder heaters put them about 8" or so above the chicks. I'd think starting out above fully grown chickens could be about 18" but again, that depends upon temperature as if it's 20F, that would be too high.
Debbie's idea of hanging it on adjustable chains is a good idea. The owner of Sweeter Heater told me "I would think 18-20 inches above them would be sufficient. If they act cold, just lower it a bit or vice versa." I did end up lowering it for my ducks.