how big should a horse stable be?

when you mean stable do you mean stall...a stall depending on the size of a horse should be any where from 10 ft by 10 ft or 12 ft by 12, the height should be taller then what the horse could hit if it were to rear up. So if you wanted to stable 5 horses your stable should be between 50 ft by 60 ft (with a 10 foot aisle way) or 60 ft by 70 ft (with a 10 foot aisle way). The height really varies but I would do 12 feet at the lowest part of the roof.

Big enough to house all your horses.

depends on how many horses you have?