Dry lots, or sacrifice paddocks, provide an opportunity to move horses off the pasture during winter months, wet or dry times, and/or time of needed pasture rest.
Dry lots can vary in size, but should provide a minimum of 400 square feet per horse. You also need to account for feeding, water and shelter space. Also, 400 square feet assumes the horses in the dry lot get along with each other.
A three-sided shed or lean-to, can provide adequate shelter for your horse. A 12' by 12' lean-to can accommodate 1 to 2 horses. As horse numbers increase, run-in shed dimensions should increase by 12 feet per horse.
For example, if you have 3 to 4 horses, you would need a 12' x 24', and with 5 to 6 horses, a 12' x 48' lean-to would be sufficient. Again, these recommendations assume all horses utilizing the shelter get along with each other.
For more equine information visit the Horse Extension website of the University of Minnesota.