Whatever the horse is happiest with! If the horse doesn't mind, then pasture, and if that is not available, a paddock.
It depends on what the paddock is being used for. If the horse is going to live there with no other turnout, I would have it be a minimum of 100' long, but although that may be big enough for the horse to play around in, 100' doesn't seem like enough for the horse with him looking at it so he probably won't run around. My two are kept in a 50' (and 70' in some spots) by 110' pen in winter and they act calm. But put them out in a pasture and there will be no grass left because of the wet ground and them running around... otherwise, if you can turnout in a larger area, any size is fine as long as it is no smaller than a stall of course.
“Good things come to those who wait… greater things come to those who get off their ass and do anything to make it happen.” - Unknown