Depends on your geographic location, climate, amount and type of pasture forage, how much supplemental feeding you plan to do.
While a horse CAN be "kept" in a small area such as a small paddock and/or stalled at night in a barn, they need daily pasture turn-out time for natural grazing and exercise. I would think that a horse would need at least one acre of pasture (3 to 5 acres would be better). Horses are "continual" grazers, eating a little on a frequent basis.
If a horse is kept in a small areaa, it won't take long before they eat all of the grass and turn the area into a barren grassless area subject to wind and water erosion--that is just their nature.
Horses need some supplemental feed and high quality hay twice daily if they are to be maintained in top condition. Salt, minerals, plenty of clean water are the other basics. They also need a substantial shelter against wind, cold rainy/snow/ice days for sure.
Stall or tiny paddock areas confine horses too much and they don't get enough exercise. Resultantly, they tend to get restless and hyper when they don't have enough exercise.
It goes partially with your horse's age. My pony is old so he can't eat much grass. It also depends on your horse's health. Like was it a abused horse so it's system cna't handle certain foods? I know a horse that can only live in it's stall because it was abused so badly. If you don't have much property like me and you're getting a senior horse, he'll eat up the grass quickly and eat tree leaves so make sure you give him a reasonable amount of feed and frequent hay. If you have a mini, you only need 1 half acre or so. A full sized horse could need 1 to 2 acres. Generally same with ponies.
I like to see more & more people get into horses! I also like to see the horses have room to run!
Sure a horse could live in a box stall & run around in a circle all it's life or live right by your house with just enough room to turn around. But I like to see horses be what they are horses & run & play! Horses can live off of hay, salt, water, & a little grain & be happy.
IF YOU DON'T WANT TO SPEND ALOT ON FEED & HAY, I WOULD GO WITH 1 1/2 TO 2 ACRES PER HORSE.
YOUR FEEDING PROGRAM HAS ALOT TO DO WITH HOW MUCH YOU PLAN TO RIDE YOUR HORSE, AND WHAT KIND RIDING YOU WILL DO.
MOST OF THE RIDING I DO IS FOR PLEASURE, AND I OCASIONALY HELP MY NEIGHBOR WORK HIS HERD. *LIGHT WORKLOAD*
I HAVE TWO HORSES ON 3 1/2 ACRES OF GOOD GRASS, AND I FEED EACH A GALLON OF 12% PER DAY. THEY STAY IN GREAT SHAPE. DURING THE SPRING AND FALL WHEN I RIDE MORE, I FEED MORE.AS LONG AS THE WEATHER IS GOOD THIS WORKS OUT FINE.
DURING REAL DRY WEATHER THOSE TWO HOSES CAN EAT THAT PASTURE DOWN IN NO TIME. THEN I SUPPLEMENT WITH HAY AND MORE FEED.
DURING THE WINTER HALF OF THE PASTURE IS PLANTED IN RYE GRASS WHICH I TAKE THE HORSES ON AND OFF OF AS NEEDED. I ALSO FEED 12%, HAY, AND PUT OUT MINERAL BLOCKS.
THIS WORKS OUT WELL FOR ME IN CENTRAL LOUISIANA, BUT COULD BE DIFFERENT DEPENDING ON WHERE YOU LIVE. IT CAN ALSO DEPEND ON THE HORSE. I'VE HAD HORSES THAT WERE EASY KEEPERS, AND I'VE HAD SOME THAT TOOK TWICE AS MUCH FOOD TO STAY IN SHAPE.