You can place your horse on an acre or less of land and be just fine raising the horse as my uncle had a horse that resided in his backyard with less than an acre and it did just fine but he took it for walks around town for exercise and rode it around town as well.
However it's preferable to have at least 2 or more acres to allow the horse to run in a fenced in area for constant exercise and for the horse to be more happy.
I would personally recommend at least 10 acres of land for the horse to be able to run around like it needs too.