How much space .....?

12 x 30 ft is a good shelter for a pasture, only. I don't know about laws, but I will tell you my personal opinion. Generally, the rule of thumb is 1 acre per horse. I personally like 2 acres per horse, and if the grass is thin, I like 3 or 4 acres per horse. If this amount of land is not available, you can stick to the 1 acre per horse, and do not feed directly on the ground. That cuts back on some of the water hay, and the ground will stand up longer. When it rains, either keep your horse inside, or have a dry lot on hand for the wet weather. Hope this helps.