If you have limited space, consider a three-sided run-in shelter. Even in severe weather, horses will do fine. If you will have 2 horses, you will need one built that is at least 12x24 feet. By puting a panel down the middle, you can always convert this setup to 2 stalls.
A limitation of small stalls is there is no room to groom or tack up your horse, and no room to store grain and hay.
If you get plans for a run-in shelter, a local carpenter can easily build it for you.
A barn can be as big and elaborate as your space and budget permit. Some even will install mirrors on the walls, and provide a heated viewing area. There are equine barn-builder specialists who can lay out the multiple stalls, tack rooms, washrooms, sprinkler systems, and overhead hay storage. These builders advertise in local and national horse magazines. They also know how to safely install light fixtures, and know just how high and wide the doors need to be, and provide for proper ventilation.
Dirt floors are fine, but you can't wash them down. Wood floors are also fine, but be aware that over time, urine and manure will rot the floor. Concrete is hard on a horses legs, so it should be covered with rubber mats and a deep layer of bedding