My runners don't need me to make nests - they make their own. They like corners, though.
Sizing depends on the size of the duck - Pekins are pretty big. Also to consider is how much time they will have to spend in the shelter during inclement weather. Up here, that could be days on end. So enough space and an area for food and water (some use a section of flooring covered with half inch metal hardware cloth to prevent the bedding from becoming sodden.
About 6 square feet per duck would be a starting point to consider.
3 feet high minimum, and consider your own needs for getting around for cleaning, egg collecting, getting ahold of injured ducks.
