You could technically keep a horse on a small under acre of land but to give the horse plenty of room roam about you should have between 1 to 2 acres of land minimum for the horse to be happy.
However if possible more land for the horse is even better. I personally wouldn't have a horse unless I could have at least 1 acre of land for the horse.
I would though prefer to have 5 to 10 acres so the horse has way more room to spread out.