Paddy's wee yard has about 3.75 acres for 2 x nf ponies and him (a connie). They live out 24/7 365 days of the year. The field does ok but for 1 month in spring and 1 month in autumn they get sent up the road to a friends field to rest the land.
If your horses are coming in at night and you can manage the land well it is do-able but quite challenging.